UK tax on foreign capital gains

Yes they are. As this gov.uk page on Capital Gains Tax states:

Overseas assets: You may have to pay Capital Gains Tax even if your asset is overseas.

Normally you'd declare them along with any UK gains in your Self Assessment's capital gains section. However if you've somehow already paid some tax on gains on them abroad then you'd need to also use a section of the SA106 "Foreign Pages" to claim relief on that. (Of course if the foreign shares yielded any dividends, you'd probably be filling out SA106 anyway to declare the foreign income and claim relief on withholding tax.)

The self assessment pages relating to capital gains are SA108. So far as HMRC is concerned there's nothing particularly special about the treatment of foreign shares over UK ones. Obviously you have to convert to a GBP value, but the same ideas of chargeable events and thresholds for reporting and tax-free amounts and so on apply. The main complication is probably the fact they might have been taxed a bit in their foreign jurisdiction already. (Where things can get more complicated is with offshore funds/ETFs which don't have "reporting status".)