I'd imagine that BTC asics would be your best bet. Hoover up the ones off ebay, cos nobody who's paying for power can run them in the UK anymore. Or order new from Sh**main.

The new algo asics are in high demand and subject to forks and all sorts of malarkey, making them higher risk. A barnload of S9s will gradually turn a profit for very little outlay, and need little maintenance vs gpus.