You would have thought wrong. Swimming has a lot of demand on your mobility, and swimming wet suits are highly designed bits of technical kit. Good ones cost around £400. £100 is entry level. Seaside shop special is okay for paddling, not for swimming hundreds of hours in. It's not about warmth, it's about mobility, float and (absence of) drag.
As to your other point -
Wiggle (amongst others) have an excellent returns policy so if you get the wrong size you send the old one back free of charge and get the next size up (down).
Yeah, wetsuits for swimming generally (or should) have more mobility around the shoulders and underarm region for obvious reasons. You don't want to try and swim in a surfing wetsuit for example because you will fatigue a lot quicker due to lack of mobility in the shoulder region. Additionally, more expensive wetsuits will have varying neoprene thickness levels (e.g. thicker on leg region) to help with those who have 'sinky legs' when they swim (often cyclists due to larger muscle mass!). Also, the top wetsuits might have 'catch panels' on the forearm region to help with the catch phase of the swimming stroke. Also, those that are more triathlon specific will have tech to assist with quick removal of the wetsuit in transition (although chopping a couple of inches off the legs and arms and a liberal coating of bodyglide around wrists/ankles helps here).
For my first foray into triathlon earlier this year I just bought an entry-level blueseventy wetsuit (the 'sprint') for £100. Got it in a sale and have been happy with it. I also just bought it from Wiggle and went on the sizing chart - but in reflection I'd rather have gone somewhere to try them on and get properly fitted. Mine fits fine, but probably should be a bit tighter for a proper fit. A lot of people buy a wetsuit online using the sizing chart and then when it arrives they believe it's too tight - when more often than not it's probably the correct fit - they're just not used to how tight it should be. It should take effort (and time) to fit into a wetsuit and as a rule of thumb I believe you shouldn't be able to pinch/pull away the wetsuit from the small of your back - but equally you should have mobility in the upper body/shoulder region.
Top tip for getting into a wetsuit - use carrier bags. Yes, put your foot (one at a time) into a carrier bag to slide down inside the wetsuit. The same applies for your hands. Just stops your skin snagging on the suit and helps reduce risk of tearing. Additionally, cut your nails as neoprene can be easily damaged. Fitting pictures can be found
here.
Good luck buying a wetsuit - the Orca S4 looks nice, but take a look at other brands and shop around on price! Also, I really would recommend trying some on if you can!
but might have to give them a go as I'm sure they would be really advantageous for sports!
