The Ofcom site is out of date, I can only say where the Orange / T-Mobile cell sites are in that area.
You need to move away from the dongle and look at a router that supports the sim slot and a external antenna point at your chosen cell site.
(I don't know of '3' are on the same cells but it's worth a shot.)
I might be wise to look at hardware that is also 4G capable when it comes to you, if your budget permits.
- 3G / 4G routers. - Other hardware suppliers exist etc...
- You want high gain, but to keep the antenna cable length as short as poss.