first of all, you talk about upgrading.
this can't be done until you're in WDW. So this merely writes off any saving on accomodation or car hire (you'll probably be better off booking the Brits guide rate with alamo)
you will have a Ultimate Ticket, which means that if you use any waterpark, disneyquest (2015 tickets) or minigolf, you'll have to upgrade to a PAP which is nearly £100 more than the regular AP
if you don't use the waterpark option you can get a regular AP, but you'd have to pay for waterparks or minigolf
it pays for itself if you stay offsite, as 14 days worth of parking is roughly £160
note that not all places will give you the same quote for the upgrade. Read the various threads about upgrading to AP on this forum, and see that it's not just a matter of calculating the difference in price between the Ultimate in GBP and the AP in USD, it's more complicated than that, and depends on how the CM does the calculation.
it's totally worth it if you upgrade only one ticket AND stay offsite (parking charges)
if you stay onsite, then it's debatable, since to break even you'd need to spend roughly £1500 on merchandise