Disclaimer: it's hard to be definitive as there may be some law or tax rule I'm not aware of.
From a UK perspective, this should be perfectly legal. If it's just a one-off or occasional thing for personal reasons, rather than being done in the course of a business, there probably aren't any tax implications.
In theory if there's an identifiable profit from the transaction, e.g. because you originally obtained the INR at a lower exchange rate, then you might be liable to capital gains tax. However this is only payable above approximately £10K capital gains (see http://www.hmrc.gov.uk/rates/cgt.htm) so unless this is a very large transaction or you have other gains in the tax year, you don't need to worry about that.
I would only recommend doing this if you trust each other. If one side transfers the money and the other doesn't, the international nature will make it quite hard in practice to enforce the agreement legally, even though I think that in theory it should be possible.
If the sums involved are large, you may find that the transaction is automatically reported to the authorities by your bank under money laundering regulations, or they may want documentation of the source of the funds/reason for the transaction. This doesn't automatically mean you'll have a problem, but the transaction may receive some scrutiny. I think that reporting typically kicks in when several thousand pounds are involved.