Now my problem is the minimum withdrawal amount allowed by Coinfloor is 0.005 BTC. I only have £50 in Coinfloor =0.001842 BTC
There is nothing you can do about that. You should've read their FAQ before depositing money.
You can try and look whether one of these exchange have better limits (for the future):
https://www.buybitcoinworldwide.com/united-kingdom/