when i first joined the scheme thats what i did, as comfortablynumb says they will probably be low on the totum as far as an offer on your pex is concerned. thats a decision only you can make, in my case it wasnt worth a lot anyway and webuyanycar etc or a private sale wouldnt have got me much more so it was worth the conveniance of tipping up on the day and swapping. they gave me a cheque there and then for the difference.
also if you dont need a factory order you may find one in stock it may be worth asking, back in the day my first motability car was a citroen and when i went through the order the girl found one on a ferry on its way over but not the colour i originally wanted but she could give me a choice of 3 so a compromise on colour saved 10 weeks wait. i picked it up 10 days later.