How to be certain a Boardman ADV 8.8 would fit without prior road bike experience?

This is why people recommend not going to Halfords. (For the benefit of people outside the UK, Halfords is primarily a car accessory shop, which also sells bikes).

If you go to a proper bike shop, the staff will insist on fitting the bike to your properly, rather than coming up with excuses not to. They'll also let you take the bike on a proper test ride, rather than just riding around the store.

Answering 'How to be certain if a <insert bike model here> would fit without prior road bike experience?

You have to rely to a large extent on bicycle shop staff advising you, taking to account what kind of riding you want to do as well as your height and relative proportions.

If going to a store like Halfords, it's a good idea to arm yourself with some general bike fit knowledge. It's relatively easy to find that online.

You probably will feel a bit 'bunched up' and that your knees are coming up too far if this if your first drop bar bike and you are used to hybrids with a more upright riding position.

Know how to ballpark the seat height: heel should touch pedal with butt on the seat and leg straight. When seated you should be able to get toes on the ground. It's good to set the saddle fore-aft position too if they will let you: line from kneecap to pedal axle should be vertical.

You then look at what angle your torso and arms are at when on the tops, hoods and drops. More leant forward is more aggressive. If this is your first drop bar bike you probably don't want a very aggressive position. I'd say back no more than 45 degrees from vertical and angle between torso and arms about 90 degrees when on the hoods where you will spend most of your riding time. (Commenters, please feel free to add better advice if you feel I'm off base here).

It helps to have a friend with you to eyeball the angles while you are sitting on the bike. Remember that you can swap out the stem to make a couple of centimeters of adjustment in hand position.

There is a lot of psuedo-science (and granted, some real science) around bike fit.

My experience is this - there is a LOT of adjustment available through seatposts, saddles, stems and bars that means that the actual frame itself is not as critical as people would like to make out.

If you look at the bikes ridden professionally, they often have pretty extreme setups that nobody would buy "new" from a shop. This is because, fundamentally, everyone is different.

I would suggest buying a frame size you won't regret. And basically for me, that is standover height (flat-footed). If I'm going to squash my particulars every time I have to get off the saddle at a junction, I'm not going to enjoy the bike.

I'm inclined to go with the size that Halfords suggested. The XL may be uncomfortable. Ignore theoretical calculators, because there is no ISO standard for frame measurements.

As you found out, a huge part of cycling mechanics is the saddle height, and once you have got that right, you will find that there is plenty of alternative stem heights available to get your back at the angle you feel comfortable at.

I would also caution that drop bars are not for everyone. The bar type is a bigger commitment, because it is not straightforward to switch from drop bar gears and brakes to straight bar gears and brakes.