philip - you need to do more 'reading' - suggestion (4) by 'Bealzebub' was not meant purely as a joke! Quote 'You are confusing two different systems'.
AFAIK, in the UK, GPS is not certified as a 'sole means of navigation'. This means that even with the best GPS in the world, you need the INS system to fly legally. Without it there are severe restrictions on what you can do. That is why it is 'aligned' before departure, indeed using the GPS position to do so (after confirming it is correct). Yes, GPS is accurate to a fine degree (assuming, of course, the signals are not interfered with as with the US military GPS system). The INS has at its heart an IRS system which gives attitude information amongst other things, which GPS cannot.
All these definitions are available on Google' and the 'Tech Log' forum has some useful links at the top.