I think Sipgate are the cheapest at £9.95 per month for unlimited calls to UK landline & mobiles. But they don't have a native smartphone app which allows you receive & make calls on your mobile using the same VOIP plan.
I appreciate you said Sipgate don't have a native
app to use their VoIP on a smartphone, but it's worth pointing out (as no doubt you're aware) that there are free third party apps which do the job very well, and having the advantage of being configurable with multiple VoIP accounts from different providers (including Sipgate).
e.g. I used to use Zoiper but following an update which I didn't like I now prefer CSipSimple which I've presently got configured with 4 VoIP accounts including 1 from Sipgate. I don't use VoIP on my mobile very often so I haven't bothered to find out if using dial plans is possible with CSipSimple, I just select the account I want to use manually which is simplicity itself.
Something to be aware of when configuring the same VoIP account on multiple devices is that some accounts do not allow this, which can result in the registration of the account flipping between the devices, i.e. as a device renews its registration, the other device(s) will lose theirs. Sipgate does not fall into this category.
FTTP 80/20 Mbps