Some manufacturers give a GmFV to allow a deposit for the next car. Agility seems to give a GMFV that will allow you to give the car back but unlikely to have any equity for the next car. So if you put say 4K in your monthly will be lower but at the end the 4K is gone.
But if you don't put the £4k down as a deposit (say just £1k deposit), then your monthly payments will be higher. So like I said, you've paid exactly the same amount over the term.
What you pay each month for a PCP is:
(List price -discount -dealer contribution -deposit -GMFV) / term.
If you're offered over the GMFV, it is likely to just be the dealer playing games when you buy your next car (the same way some some dealer give you a sky high prices for a PX). You're just getting shafted elsewhere - I believe it's known in the trade as the '4 square method'
Personally, I budgeted on getting absolutely nothing over the GMFV. That way there can only be a neutral or positive outcome for me.
Setting a GMFV is more of a risk for the lender. If they put it too high, then the punter will pay less in interest.
If they put it too low, then they will lose out as punters would buy the cars up then immediately sell them on.
If a dealer is offering you over GMFV for your new purchase, the chances are they're just taking it off any discount they would otherwise have given you. The whole "you'll have equity at the end" is just part of their sales pitch for them to sell you a car and finance.