If you are not making outbound calls, then Andrews & Arnold are your cheapest option (£1,44 pcm). They now charge £12 one off off setup (used to be free) and £12 for porting in an existing number.
https://www.aa.net.uk/voice-and-mobile/prices/
You would need to check with them whether you have to pay for setup if you are porting in an existing number.
I don't know whether you can configure your Sky router to be used with an different VoIP provider, so you would need to get either
* a SIP phone (could be wired or DECT phone), or
* a SIP terminal Adapter (e.g. Grandstream) to plug in your existing DECT base, or
* a broadband router that can do everything (SIP and DECT), e.g. AVM fritzbox
No need to worry about "locked" equipment, as long as the equipment allows you to edit the SIP account details, you can use it with any VoIP provider. Best to buy your equipment on the open market (e.g. ebay) rather than from a VoIP provider, that should be cheaper as well.
Unfortunately, setting up VoIP is not quite plug and play but not too difficult either, a lot of people here and elsewhere who should be able to help (if you provider can't/won't help) .