From what I remember when I researched the same thing :- The signal booster thingies are definitely not legal as in effect by using them you are creating a mini ‘cell’ and therefore would need a mobile operators licence according to Ofcom – which you can’t have as you aren’t a mobile operator.
The weird state of affairs is that they are not illegal to sell but they are illegal to use – hence the number of them that you see on the web. So a non starter if you want to stay legal. (whether anyone has been prosecuted for using one I have no idea)
However the various options of boxes/apps to call out using your internet connection work pretty well, on the proviso that you have a reasonable internet connection in the first place.
I use the Tugo app thingy from O2 and it works reasonably well, but I do have a decent (fibre broadband) internet connection. Not tried it on a slower link.
Have you looked at EE coverage – in my experience it is often better in areas where O2/Voda aren’t any good?