YES , there are lots of colleges in UK which do not need a work experience (ex Coventry , Middelesex,Northampton,Liverpool etc ). However in such colleges 1-2+ work exp always helps immensely in your application process.BUt they are NOT AMBA colleges . Be careful when applying to such colleges.

 Secondly , AMBA colleges have a minimum pre-requsite of ATLEAST 3+ years work exp . All good intereationally recognised colleges follow AMBA accrediation standards. so i guess if u want a good college degree u need min 3+ years exp. However nature of work , profile of work, ur learning curve, contribution to company are all important factors looked for in the candidate's portfolio by the colleges.  is also important , COlleges like Manchester , Exeter , LSb etc requuire atleast 7+ years of exp with "managerial" expertise..

Finally , ON GMAT front ... each college in UK  has its own independent take on GMAT criteria. Please contact admission council with your profile they may/maynot ask u for GMAT score depending on ur profile. GMAT is only a filtering criteria may/may not be a definitive criteria. 

GMAT score of  550 is basic score. However some top colleges may expect excess of 700-710.

u have to give in your completed application package which includes the filled applocation form, LOR, SOP & essay by September. So ideally u shud be over with the Exam by March, so that your GMAT scores reaches the colleges.

And one more thing...u need to have 2 yrs plus work exp to apply for most b-school in US. I am taking my GMAT in Feb.

let me know if u need any more info on prep tips and stuff...:)