Exchange rate for selling foreign assets

What fx rate is used when working out gains on sales of a foreign asset?

Example:
An asset is bought for 50,000 GBP in the UK, the exchange rate on that day is 64
It is sold some years later for 100,000 GBP, the exchange rate on that day is 44

Is the exchange rate used for the day of the transactions?

So buy at 50,000 GBP = 3,200,000 THB @64 
Sell at 100,000 GBP = 4,400,000 THB @44 
Gain = 1,200,000 = 27,272 GBP

Or is everything on the current exchange rate (44 in this example)
Buy at 50,000 GBP = 2,200,000 THB) @44
Gain = 2,200,000 =THB = 50,000 GBP

Also are the any other allowances etc beyond the normal personal allowances and 0% band?
Like the expenses to acquire the asset? Or in the case of land/houses mortgage interest, expenses, capital improvements? 
 

Category 1,2,3 income has a 50% max 100,000THB deduction (salary, pension, goodwill etc)
Cat 4 (interest), Cat 5 30% (rental property 10% other property), Cat 6 10-30% (professional services), Cat 7 60% (construction services), Cat 8 60% (business, commercial, farming).

Could a capital gain come under Cat 5 30% if its property (a house/land) or 10% for other property (a painting for example) 10%, or is it Cat 8 business 60%)?

Thanks,