SC
It is currently illegal to use a GPS as primary form of navigation in the UK.
That argument doesn't stand up against those of us who have BRNAV (mandatory above FL95 IFR in Europe - and that includes UK & Eire airspace) approved units like the GNS430/530 and KLN94 series - these become the primary mode of navigation.
Still think it's illegal for primary nav?
As IO540 says - there's no point looking in the ANO because there is no legislation, in much the same way that it doesn't say if navigating VFR you're not allowed to use VOR's and NDB's - you have to use a chart, stopwatch and compass.
You can navigate anyway you like, provided you fulfill the equipment requirements of the type of flight being undertaken within the flight rules and airspace being flown.
THAT is the only criteria that covers the legal/illegal criteria.
It often pays to check the detail before making inaccurate and sweeping statements.