check out the following links:

http://www3.imperial.ac.uk/pgprospectus/studyzone/registrationforhigherdegrees

http://kcl.ac.uk/graduate/progs/qual/research.html

http://www.ucl.ac.uk/prospective-students/graduate-study/application-admission/general-entrance-requirement

well actually imperial, king's and ucl all require a masters degree before you can study for mphil/phd. I suppose it varies depending on where you apply, so if you are considering one of the more prestiguos universities make sure you look on their website for information. You could also phone them to ask if they would accept your lab experience in lieu of a masters.

good luck

Final point to remember is, Top UK students are competing for the same projects as yourselves especially at prestigous/russel group universities, i'm not saying dont apply just keep your options open, especially seeing some UK universities with bad reputations also have departments which are very highly regarded and vice versa. To check a departments research quality look at their RAE scores 4,5 and 5* are the top three. To check their teaching quality look for QAA scores which are generally out of 24.

best of luck and remember the best place to live and study in the UK is definitely central london.

I hate to say it but their are very limited opportunities for funding if you are an international student, but if you have good results (experience always helps), you should definitely contact potential supervisors or search for projects on-line. What i have been told is that one of the biggest mistakes PhD applicants make, is not to contact potential supervisors. Even if you have no questions it still shows enthusiasm for the project.