@syntax Here are a couple I've personally used, but I don't know specifics around UK numbers. These are NOT endoresements, and I haven't analyzed their privacy policies.

SignalWire: BY far the easiest to get up and running and cleanest interface.

VoIP.ms: The website is old fashioned, and they don't support MMS, but they had good call routing options.

If you want full call and MMS functionality, you have to go with Broadband or a reseller. Pinger app (aka TextFree) MIGHT work with Lineage?