Very surprised they wouldn't adjust the bike for you. When I went in they adjusted the saddle, pressure in the fork (MTB), height of bar and even fitted a different stem for me. Pretty much whatever I asked for.
Maybe different stores have different policies.
I'd be happy to pay a deposit in case they thought I was going to run off.
Not even changing the seat height for me isn't very good though. But gutted as I really liked the Boardman but I guess I'll have to look elsewhere.
Might try another Halfords to see if they'll adjust the seat for me to try out.