The broadcasters (and the companies who own the transmitters) transmit their signals on the assumption that viewers will be using a good quality aerial, like this one
http://www.maplin.co....e.aspx?ModuleNo=22523
They also assume that viewers who are in a 'fringe' area will have an aerial like this:
http://www.maplin.co.....aspx?moduleno=221124
Using an indoor aerial usually amounts to little more than sticking a bit of wire into the back of your telly. It's hardly surprising if you can't get decent reception.
Given that you get at least some Freeview reception, you MIGHT be able to improve things by using this indoor aerial (which incorporates a high gain amplifier), preferably together with your existing signal booster:
http://www.maplin.co.....aspx?ModuleNo=115127
However there can be no guarantee of good reception until you install a decent external aerial.
Chris