I dropped on a special "Unlimited" deal with Smarty for which I pay £16 per month. I have just checked though and it's back to its normal price of £20. Even at that price it's still excellent value as it really has no useage limit, caps or throttling in the UK. I use it in a TP-Link M7650 4G Mi-Fi mobile router. Smarty uses the "Three" phone network and coverage is excellent offering 3G & 4G. It streams 4K movies effortlessly.
However, it's worth bearing in mind that most if not all phone companies put a cap on data useage when the SIM is used abroad. Smarty's cap is 12Gb per month. I found this out while in France for three weeks in June. It's easy enough to top it up though if you should run out.