I know nothing about timeshares or property values in the UK. Nonetheless, I'll offer the following observations and opinions which are probably universal in nature.
1. Timeshares are almost never a good "financial investment". Their actual "value" lies in the use and enjoyment of the property (or properties, plural) and you should never expect to recover anything more than a tiny fraction of a developer-direct purchase price (if anything at all) in any subsequent resale --- regardless of whatever nonsense some commission-hungry sales weasel may have spouted forth to the contrary.
2. Your particular "sunset" date is apparently 35 years away. Accordingly, the fundamental question you should ask of yourself is whether you would potentially get 35 years of use and enjoyment out the purchase, considering whatever amount it is that you are paying out of pocket now, as well as the certainty of annual maintenance fees which will likely increase, at least slightly, each and every year.
3. If you're thinking that there is a big "payout" waiting at sunset, rest assured that you are likely quite mistaken in that belief, regardless of the property's value. Lawyers (and HGVC) will likley eat up a huge chunk of of the dissolution costs and hefty taxes may apply as well. Don't plan on a huge windfall, in any scenario.
Food for thought, since you've asked. Good luck with your personal decision. If you decide to rescind (cancel), don't delay in doing so.