to bad you need to keep it in the car thought (take it to work or other houses)
still think UMA is an cheaper solution some android phones have it, the Samsung S2 has it, but they should state that its not compatible with all wireless routers (you should always test it make sure the call stays on for more then 5 seconds {call 150 or 333} and that UMA stays on (
http://shop.orange.co.uk/mobile-phones/pay-monthly tick Signal booster)
what Three need to do is put up more masks and make sure they are not so far apart from each other (3g hates walls) in housed areas they need to be less then 1-1.5 miles apart or even closer as the Three tower from me is only 1mile away but there are 15-20 houses between me and it so the phone randomly works even thought it has 2 bars but that means nearly loss of signal on android phones (i been using giffgaff that uses O2 and they seem to get mask placements mostly right or have More dense 3g network as i norm get good 3g indoors or they are using the 800/900 band for 3g as i have seen on wiki, so less Dead spots), there is an Orange tower that would love Three to be enabled on it (t-mobile and Three seem to be using the same mask that has the signal issues where i am)
i just put my Three sim back into my HTC desire (£15 all-you-can-eat add-on) see if they fixed the coverage around here, but they have not and speed is not that grate as well avg speed is less then 1.5mb i had issue with youtube stalling some times as well, i even got an txt mesg that was an bit late as well (my brother has Three phone says its not that reliable has signal but can't use calls or internet randomly)