Well....if you might want to use the GPS unit for IFR ops in the future (provided of course the UKCAA ever gets off the dime and allows same for IFR approaches) you need a panel mount unit, approved for such ops.
The Bendix-King KLN89b and KLN94 are superb for this, and quite reasonably priced...especially used 89b's
Having used a King KLN 89b for over ten years in my private aeroplane, it has supplied very reliable and accurate service.
If you want to go used, JA Air Center in the USA has KLN89b's (and a few KLN94's avialable from time to time, fully serviced and yellow tagged, and...the price is right.
J.A. Air Center - DuPage Airport - West Chicago, IL