Aircraft have been the dominant weapon at sea since the early part of WW2, and the principle was even worked out before the war began. Thats still the case although I'm sceptical about the longevity of aircraft. Failing that, submarines are the best bet, but they are vulnerable to interdiction by escorts. That said some of the modern boomers are very quiet indeed.

Anyway, all this stuff about "oh you might need an army all of a sudden" is largely wrong. On the basis given, the American states should have armies just in case another civil war breaks out, and we should have one just in case the Pope orders someone to impose catholicism again. Times change; things that happened in the past do not automatically repeat themselves. There is no prospect the UK will go to war with France or Holland again, let alone Belgium, and so the Channel coast is secure. There is no prospect we will go to war with any European state without a long process of detriorating relations which would give plenty of time to re-arm. The most that can be said is that we should probably keep a few destroyers about and that is the limit of need. In fact if we were ever involved in a serious war again it weould almost certainly be in conjunction with the whole of the EU.

The other purpose of our army is, as has been observed, essentially political chest-beating, which I can frankly do without.