Call the shops you can get to and ask what they can do, even better go in person as then they know you are at least semi serious.
Just because that's what's in the generic advert doesn't mean they won't be flexible. None of the retailers are Starbucks or McDonalds, they are small enough to be able to make decisions themselves.
Otherwise, then see what your local bike shop can do for £400ish, and get a £100 wetsuit from elsewhere.
But if you try buying a bike and wetsuiit without someone there doing a fit then you'll be highly unlikely to get the right size. Most people go too big on both when left to their own devices.