The problem with the £300 one is that it doesn't tell you the azimuth (heading) of the conflicting traffic.
To get the one which gives you some idea, it is the £1000 one.
But all these are - for paperwork / regulation reasons - portable cigar-lighter-plug type things which IMHO put a lot of people off because you end up with the power lead strung across messily the instrument panel.
If one could fit the £1000 one somewhere neatly, that would be something else, but there is no legal way to do it because it is not certified for a permanent installation (not in a G-reg, for sure).
A proper installed system starts at around £10k, but is rendered much less than useful by the number of planes whose pilots are exercising their free citizen's privileges and flying with transponders turned off.