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.