I reckon if you search around a bit, you could probably find a combined GPS plotter and fishfinder for not much outside your budget (but then you'll have to start saving again for the maps!).
Search on ebay/internet - be careful though:
(i) some of the units advertised cannot take external cartridges, and the 'background maps' are only useful for the US
(ii) be careful of 'GPS ready' fishfinders - which require add ons to become plotters... what you require is a combined plotter/fishfinder
(iii) GPS units sold from the US may only have a background map for the US - which will make the unit a bit useless as a plotter until you buy the Map cartridge. I would still work as a fisfinder/depth sounder, and basic GPS unit though.
(iv) check what type of cartridge the unit takes (ie Navionics, c-Map) - then check the catalogues of these cartridge map suppliers to be sure that cartridges are available with the correct level of detail for the areas you are interested in..
Probably a good idea if possible, when buying off the internet to buy from a source as close to home as possible (ie UK based...., better shipping times, UK compatible gear, support you home economy, and also for some odd reason - maybe a better feeling...). I bought a Lowrance 320DF (combined GPS plotter and fishfinder) for something around you budget (I forget exactly...) from a UK supplier on the net last year - works great.