A 5" screen chartplotter with sonar and dual frequency transducer, complete with the entire British Isles charts should do the trick. About 500 quid:
If you can stretch the budget to over 500 then the same size unit comes with a high resolution screen, the 556S.
Were you to want the option of upgrading to to broadband radar and downscan sonar then a Lowrance plotter might be worth a look, but for sheer value and ease of use the Garmin wins hands down.
Let me know if you want some exact figures.