Office Job for Someone Without UK Office Experience - LONDON?

WHY? I simply don't understand why can't I get an office job if I don't have UK office experience. I am fluent in English and speak well and can type, can use the PC but agencies telling me that without UK office experience they cannot help.
I am happy to start on the very bottom: stuff envelopes, do filing or photocopying all day.
I was a Bar Supervisor for a year, but prior to that I was working in an office for 5 years in my home country.

Any ideas or suggestions what should I do?

Do voluntary office work for charity?

So why are you so desperate to work in an office? Why did you give up the bar supervisor job? Could you not work your way up in that job? The money ends up being a lot more than doing an office job, and is a lot more interesting than suffing envelops all day.
Qualifications are hugely important in the UK. Get an ECDL (European Computer Driving License) ASAP. If you really know what you're doing with office IT, it'll be tedious and boring, but do it anyway. Further Education offers it, as well as various charities and other sources. Look into what else FE has, like an NC or ND in Office Administration.

Some places can't be bothered to check overseas references, so if agencies won't, try elsewhere. Offer them the option of e-mail and fax contacts as well.

Get a reference in the UK soon, even if it means taking a lousy job. 'Tis the season, so perhaps part-time seasonal retail? At least no one will question why you're leaving it in January. Carework is another sector that often hires a lot of people from other countries, you might have better luck there.
If that is what you have been told by the agencies, then you are speaking to the wrong ones.

Might I suggest you peruse to my Job advice site for starting ideas on getting jobs. Start with your CV as that tends to be the worst thing that Jobhunters mess up on.

Look in papers and Recruitment sites for any of the following:
Junior Receptionist
Office Junior
Junior Administrator
Typist

It is possible to move to a different work sector, so keep going.
G!!; Business Recruitment Consultant