I use a booster, we have no signal in the valley that we live, in the garden in certain places I can just about muster 1 bar!! The booster works ok, I have the aerial on a pole on the outbuilding, inside the house I generally get full signal (not 3G though as wrong frequency for that) although the reception can still drop calls sometimes, but at least you can get them in the first place which you couldn’t before!!
As for it being illegal in the uk that is wrong, whilst uk law states that mobile phone transmitters have to be licenced you will effectively be operating under your network operators licence therefor totally legal, if you are at all unsure you can always ask them for permission to use a booster (they will happily supply this) and you then have written conformation!
Second to rhat, even if it were illegal, assuming you live in the sticks, and no one is around and all your doing is repeating a signal around your own home/land then there’s no way on earth you’d get caught!! But like I say, I have investigates this and whilst some argue otherwise it is most certainly totally legal and if you have no/little signal I would highly recommend one
The one I brought when we moved In September was second hand off ebay and was only a cheap china import type one, it died last week after a power cut so I would recommend a better quality one perhaps!! I shall certainly be relaxing it with one now I know they work!!