as my tablet was starting to play up  ive recently bought a alldocube t12 10 inch tablet which has a sim slot. about 60 quid on fleabay,   ive stuck in a 3 12 gb 12 month sim. it will also tether the lap tops and the wifes ipad.  will use it for a while to test out the signal and how much data we actually use.    we pay bt 3 monthly so end of feb is decision time, at the moment  looks likely that bt will get the chop. will be a bit of hassle changing our business phone numbers but not insurmountable.
my mobile phone is on a plusnet 5 quid a month contract,  both our phones are dual sim so we could have 2 business numbers and 2 personal for 20 pound a month with 200 minutes on each, far more texts than well ever use and a tiny bit of data all on ee.   
also got the ee 
mifi on 4gb a month in case the 3 signal aint there.