It's simple; if that miner that cost you £2,800 only mines you 1 BTC before end of life and the value of 1 BTC rises to £20,000 sometime in the future, then it was worth all the effort.
For £2,800 you could buy 7.57 BTC (@369.84BTC/GBP right now).
If BTC rises to £20,000 (!) you would have £151,414.61. I think £151,414.61 is greater than £20,000.
Buying miners is for the fun, not the profit. Or you are dumb.