I seem to remember that UK schools taught that we have a maritime climate, though I forget what they reckoned that meant.
A significant element is said to be the Gulf Stream, which gives us milder winters than continental places of similar latitude, but also cooler (and damper) summers than those same places. A sort of moderating effect overall, and it seems widely believed that if it were not for the Gulf Stream our islands would be a less pleasant place to live.
I hadn't previously considered our climate as semi-tropical (it seems pretty unlikely here during the winter on the north-east coast of England) but any term is likely to mean different things to different folks.
There's a large wikipedia entry here which has much to say on the subject of UK climate. Last summer felt pretty feeble, and this winter a bit harsh, but it would need some serious study to say whether this is more than just an anomaly.
I wish I could remember who came up with the lovely view "Everybody talks about the weather, but nobody actually does anything."