Uk user here
I use the following (in order of most traffic sent and shows the primary use of each ITSP)
1.
sipgate.co.uk - all incoming calls, outgoing UK landlines, 999 (aka E911), some premium numbers
2.
localphone.com - all outgoing mobile calls, freephone, some premium numbers
3.
voipfone.co.uk - some premium numbers, 999 (aka E911)
Configured as a trunk group so I have failover between providers so all 3 configured to offer freephone, mobile.
localphone are cheap to test as you can credit £1 by Paypal so not much lost of it does not work out. The other 2 have £10 initial minimum while sipgate basic offers a free incoming number (DID)