Just looked on amazon UK and they are selling a cel fi unit for £99.99 and small delivery charge.
If you get a good signal outside on 02 and giffgaff as they work with mvnos that use 02 checked on there own website and giffgaff is mentioned to work with this.
You basically put main unit in window were best outside reception is then the second unit in area of house with bag or no reception and it repeats the signal to cover all the house.
It is the only uk legal repeater that is allowed to be used in uk as ofcom say it does not interfere with the network.
I have put a picture of the unit on amazon UK on here to help people to decide if it will help them. It does not require broadband to work neither.