They setup contracts with one another.
A good firm I have worked with in the U.K. is Concentra.co.uk
They work with a firm in Belarus called iTransition (itransition.com).
Your best bet, in my experience, is to find a firm in the UK (or US) to manage the developers in the low cost country.
Culture is also an important consideration here when thinking of communication. Belarus, I have found is more European. The time difference is less severe as well.
Russia is a good area to look as well.
If you are looking for a bigger player infosys has a global resource pool.

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