Specifically in London, you are in an 'urban canyon' situation, ie the sky view is very poor, and portable GPS with their tiny internal aerials are somewhat challenged by this.
The new Etrex 20 and 30 (and I'm not sure about the 10 which is a very basic non-mapping model, you'd have to check) are currently the only models on the market which can utilise the Russian satellites, as well as the US ones, simultaneously. In theory this should partially offset the problem with limited sky view, having twice as many sats to go at. I have a basement window which only sees a tiny patch of sky to the north, and my 30 does well for finding sats from that windowsill. [edit: - I do have other windows too!!
But I'm not at all convinced it works this way in practice - in normal use it rather looks to me as though my Etrex 30 is seeing the Russian sats OK but is preferring the US ones
Anyway, all future new models by any manufacturer, will probably share this new capability.