ive watched and tried and tried but ijust dont really sound beleivably british...i think becos my voice is too deep or im not sure..i need it for a play im doing..any advice? on any exercises i could do?

It doesn't have anything to do with the pitch of your voice it has to do with the sound substitutions that distinguish an english accent from an american one. If you cannot hear and duplicate those substitutions (like substituting the er in american with ah in the middle and end of words), there is little hope for you. For an amateur play you don't have to do many substitutions, but you have to be consistent.

Which British accent?

There are dozens...upper class Eton, lower class London Cockney, Rural Yorkshire. The line in My Fair Lady about being able to tell where you were born, within a few blocks, by ones accent is not totally unaccurate.

So if you think there is just one, that may be part of your problem. An American accent would not mix "Yall" and "Youse Guys" for example.

Go find where REAL English people hang out in your town... there is probably an expat pub if your town is of any real size... and listen to them.