No - it's a lot more subtle than that. Most blackcaps, including UK ones, migrate to Africa for winter. But (I think mid 20th C) some continental blackcaps began migrating W rather than S, taking advantage of garden bird feeders in the UK, and the milder winters in the last 20-30 years. The BTO garden birdwatch survey shows a clear peak for blackcap use of gardens in Jan-April.

The advantage to the birds is that they get back to their breeding grounds earlier. And because of this they tend to mate with other birds who have wintered in the UK, instead of waiting for the African-winterers to return. This is now being reflected in morphology, with the UK-winterers developing shorter, rounder wings, better adapted to maneouverability as opposed to long distance flight, with the possibility that in time there will be two distinct subspecies/species, one migrating N-S, the other E-W.

The blackcaps which migrate to the UK in the summer live in scrub and tend to feed on fruit etc, not needing to visit gardens frequently. But the winter migrating ones eat a much wider diet, including fat, and make a lot of use of garden feeders.

So the chances are, if you have a blackcap in your garden at this time of year, he's a winter blackcap, and will head back to germany for the summer.
The early summer arrivals probably won't be coming to your garden.