Bill,
Its because the UK has global interests that are so obvious. Just look at the pattern of trade and investments- the UK invests more abroad than other European powers and its interests rival that of the USA. Return on foreign investments are a major contributor to the UK economy.
Britain also has (reletively) extensive patches of real estate still dotted about.
Approx 10 million UK citizens work abroad.
The UK is a member of the UN P5.
Its a member of NATO.
Its a signatory to the FPDA
Thats why the UK has a navy with a global reach.
Now, if the UK's economy predominantly faced inwards, didn't have extensive economic, cultural or historic ties across the world, didn't play a major role in international affairs then yes, we could be like Ireland or Poland and just have a few patrol vessels.
Your argument cuts all ways- in those circs we wouldn't need much of the RAF either- just a few squadrons of cheap Gripens- just like the Hungarians or Greeks.