Will I be allowed residency in Tenerife?

Yes, but move before 31st December 2020 due to Brexit. After that could cause problems with added paperwork.

Could one rent a property before 31
12.20 and gain residency
I say rent in case the person wished to move back to uk after finding Spain not enjoyable

If you move to spain you can apply for residence status whenever that is.

If it is before 1st January then the amount of income / assets required will be less.   Either way you will need 100% medical cover

200,000 euros savings is way above the minimum so you should have no problems.

I plan to movw to Tenerife  and situp a business I think  that is the easy way to stay at the Island  residence is just paper work as long you can prove you can find a job and doesnt expect living of social service any one can Find a way to becone resident its just paper work and a fare involve.
Well any country in tge world is moreless the same if you not a national.

My plans am more or lesser dashed atm if not for ever Spain requirements have changed quite a bit since the transition  I will now have to have 30k plus each year in a bank account just siting there for I think 4 years to satisfy Spanish non lucrative retirement visa when the rest of the Eu only needs about 7000 euros
The uk as basically stuffed anyone with a budget going to Spain to retire unless your rich to afford to have money tied up for years not being able to touch it for the sake of a visa