I guess that depends on why you want the add on. If you want to stay at BWV in peak times, then you need to get your add on now. I bought there because EPCOT is my favourite park and I love the Boardwalk area. Its walking distance to 2 parks, Fantasia Gardens mini golf course (for ds13), a gazillion restaurants, a couple of entertainment venues, and a car rental agency in the Swolphin. Personally I don't think they could come up with a more convenient location for a future
DVC resort.
If you are planning on selling it after a few years and are looking at resale value, or if you are young enough to be planning vacations in 35 years, maybe you should wait. I know I would lose if I sold, because of a U.S. withholding tax on foreigners, so you need to consider that, since you are in the U.K.
Given that I will be 85 in 2042, if it were extended I would not pay the extra $15, unless they make a DVC retirement village (nursing home?). I purchased a 60 point resale which closed in Sept, after factoring in the closing, I figure I paid $93/pt. On Monday of this week I purchased an add on of 40 pts at $98/pt, available immediately, by the way.
I think I'm getting a great deal, since after about 5 years I'll be staying for maintenance fees. My last disney vacation (7n ASSP, 6n CSR, 3n Contemporary) cost me about $2800 for rooms alone. At any rate I'm sure I'll fall in love with another DVC resort before long...I hear Hawaii beckoning.