Don't give up on the idea just yet

I know many small tributaries of Devonshire streams that get a few sea trout in season, in some cases as early as late April but generally from mid summer onwards. These are lowland streams which although very narrow do have a number of deep (up to 4 foot) pools where the fish can hide away under tree roots. The numbers of fish are never going to be anything to get excited about, you might get one or two a season if you were lucky but they make a lovely surprise when they do show up. That said there are a few small (but not tiny) tribs where you can expect to catch fairly regularly.
The other sort of stream worth a look are the tiny ones that run straight into the sea. There are several available on the angling2000 scheme down in Cornwall that have (or had) useful runs of sea trout.
Andy