What are the findings of the ISC report into Russian interference in UK politics?

TL;DR - No more than existing, publicly available documents already show Russian interference in UK democracy.

Brief mention is made of the Scottish Independence referendum (pg 13) in that, "Credible open source commentary suggesting that Russia undertook influence campaigns in relation to the Scottish independence referendum in 2014." In regards to the EU referendum of June 2016, the report explicitly points out that the committee did not attempt to assess the impact of Russian interference.

(Note, all quotations taken directly from the Intelligence and Security Committee of Parliament, Russia Report, which has been redacted in places)

There have been widespread public allegations that Russia sought to influence the 2016 referendum on the UK’s membership of the EU. The impact of any such attempts would be difficult – if not impossible – to assess, and we have not sought to do so. However, it is important to establish whether a hostile state took deliberate action with the aim of influencing a UK democratic process, irrespective of whether it was successful or not.
paragraph 39

They did look at active attempts by the Russian state to sway public opinion, mostly through disinformation spread through social media, both organically and via paid advertising. Their findings were that this was indeed the case and that existing research in the public domain demonstrates that official Russian parties have been involved in numerous attempts to sway Western democratic processes and that the UK was a prime target for such manipulation.

...general poisoning of the political narrative in the West by fomenting political extremism and ‘wedge issues’, and by the ‘astroturfing’ of Western public opinion; and general discrediting of the West.
paragraph 29

The UK is clearly a target for Russia’s disinformation campaigns and political influence operations35 and must therefore equip itself to counter such efforts.
paragraph 31

Open source studies have pointed to the preponderance of pro-Brexit or anti-EU stories on RT and Sputnik, and the use of ‘bots’ and ‘trolls’, as evidence of Russian attempts to influence the process. paragraph 40

The committee highlights the fact that no official investigations were carried out by either the Department for Digital, Culture, Media, and Sport (DCMS, whose remit includes the internet) or by the security apparatus of the United Kingdom after these allegations began to get support and that the UK government did not take the accusations seriously until the Hack-and-Leak of DNC emails affected the US election.

We have not been provided with any post-referendum assessment of Russian attempts at interference, [redacted]. This situation is in stark contrast to the US handling of allegations of Russian interference in the 2016 presidential election, where an intelligence community assessment was produced within two months of the vote, with an unclassified summary being made public.
paragraph 47

The conclusion is that the UK government did not perceive Russia to be a threat to UK democracy prior to strong allegations of Russian interference in the US elections.

The committee concludes that the UK government ignored the threat of Russian interference until it was too late, despite hostile actions (such as the murder of Alexander Litvinenko in 2006) and continue to focus state intelligence agencies on the threat of Islamist terrorism.

If we consider the Russian threat to have been clearly indicated in 2006 with the murder of Alexander Litvinenko, and then take events such as the annexation of Crimea in 2014 as firmly underlining Russian intent on the global stage, the question is whether the Intelligence Community should – and could – have reacted more quickly and increased operational effort on Russia.
paragraph 73

Among the recommendations of the report (which include reviews into the tasking and coordination between ministers, committees, and the intelligence agencies of the UK) there are suggestions that legislation is needed to properly protect the United Kingdom's democratic processes from hostile, external influence. The committee stresses that it finds the electoral process itself is difficult or even impossible to manipulate directly, but public opinion is open to being swayed by disinformation.

We note – and, again, agree with the DCMS Select Committee – that “the UK is clearly vulnerable to covert digital influence campaigns”. In this respect, we have already questioned whether the Electoral Commission has sufficient powers to ensure the security of democratic processes where hostile state threats are involved; if it is to tackle foreign interference, then it must be given the necessary legislative powers.
paragraph 123

Most striking is that the committee could not be provided with detailed analysis of alleged Russian interference in 2016 by MI5 (the UK's internal security service tasked with counter-espionage and counter-terrorism) or by GCHQ (the UK agency tasked with communications interception, electronic surveillance and security, equivalent of the NSA) due, apparently, to no investigation being carried out (further to the point already quoted in paragraph 47).

However, it is important to establish whether a hostile state took deliberate action with the aim of influencing a UK democratic process, irrespective of whether it was successful or not.

In response to our request for written evidence at the outset of the Inquiry, MI5 initially provided just six lines of text. It stated that [redacted], before referring to academic studies.43 This was noteworthy in terms of the way it was couched ([redacted]) and the reference to open source studies [redacted]. The brevity was also, to us, again, indicative of the extreme caution amongst the intelligence and security Agencies at the thought that they might have any role in relation to the UK’s democratic processes, and particularly one as contentious as the EU referendum. We repeat that this attitude is illogical; this is about the protection of the process and mechanism from hostile state interference, which should fall to our intelligence and security Agencies. paragraphs 39,40

In essence, the committee have found that the UK authorities just don't know the extent of Russian interference, but the available evidence suggests that it existed. UK authorities don't know the impact of the probable Russian interference. UK authorities don't (four years later) have procedures in place to prevent it happening again.