Good luck with the returns process. It's a pain in the arse for the company as it means they can't resell the seat so they will fight refunding you, but you should have a decent argument under the distance selling act and the item not being fit for purpose/as advertised. If more people made this decision then perhaps they wouldn't sell them!
Let me know if you need any help choosing a different seat. I am happy to recommend options which don't cost the earth. Buying online can be tricky because it means you don't have the opportunity to test out stuff like this, but I also appreciate it's hard to get into a store. IME, out of the chains, Smyths can be very hit or miss for car seats. A lot of their stock is generic/cheaper brands although they do stock a couple of decent models. John Lewis and Mamas and Papas tend to have better trained staff and more choice for car seats, Halfords have a good stock of cheaper seats and almost universally terrible advice, but the gold nugget if you can find one is an independent nursery store near you. If you go onto the website of any "baby" brand (e.g. Britax) and look at their stockist list, see if you can spot any non-chains near you and try them out. Because they are smaller businesses the tendency is they have staff who are passionate about what they do and they really get into the details with stuff like car seats, which is incredibly helpful because as you have found, there's so much choice that buying blind is really tricky.
Some rules of thumb which will help without becoming a car seat expert:
Stick with well known brands. You know how people say with washing machines "Just buy the cheapest Bosch" ? This is the rough equivalent, IMO, of car seat to applance brands. You can absolutely buy the cheapest Britax and it will be a great seat.
Joie/Graco are budget but perfectly good seats, similar to Beko/Hotpoint appliances.
Maxi Cosi/Cybex/Britax are mid range, reliable decent brands similar to Bosch/Siemens.
Besafe, Axkid are high end, high build quality, similar to Miele.
Support leg only for spin seats, top tether is OK for forward facing only seats.
Specialise. You want your seat to perform as few "jobs" as possible so it can be optimised, so focus on the immediately needed stage and possibly the next one up - so if you're wanting a forward facing seat, then pick a harnessed seat only or a harnessed seat which converts to high back booster. If you're buying for a younger baby, preferably an infant carrier, but possibly a spin seat "up to age 4" or a larger rear facing seat "up to age 7", but nothing with a high back booster mode.
Asking a seat to protect a newborn, a baby, a toddler and a school aged child, and spin, and cost <£200 is way too many jobs for it to do any of them well. Pick maybe 4 of those things. You can go cheap by sacrificing age categories and/or spin function.
As with all rules, there are exceptions but these three will typically keep you in a good place with regard to quality and usability.