I'd go along with the County Class DLGs being pretty - a bit too pretty, though, if that's possible - alright, the Seaslug was the RN's first Missile (I stand to be corrected), but apart from two twin 4.5" guns and a big helo, they were huge ships, with large ships companies, with very little real punch.
I always thought the Darings were the most warlike looking ships we had, and had some rakish lines - though that strange curly funnel inside the lattice mast was a bit odd.
The stretched 42s were also a bit too pretty and, again, under-armed. Type 21 the same - they were built by Vospers without much thought of what weapons they'd carry.
In purely aesthtic terms, though, take a look at V Class SSBN - awesome power, dramatic lines.