And another article in The Times says this:
"Home swapping first appeared in the UK in the 1990s, partly as a tax dodge. This was because, until a few years ago, most house swappers could avoid stamp duty. The Revenue cracked down on this relief in 2003 and once the exchange has been completed both parties now need to pay stamp duty on the full value of the property acquired and register the swap with the Land Registry.
The valuation for each property should correspond to the price that they would be expected to fetch if they were being sold. If you find a match at the same price, you can do a direct swap, otherwise one party will need to make up the difference.
Any attempt to fiddle the numbers to cut your stamp duty bill will be frowned upon by the taxman. Bill Dodwell, of Deloitte, the tax cosultancy, says: “A conveyancer will need to submit a stamp duty land tax form confirming that everything is correct. The conveyancer will probably insist that you provide valuations from at least two estate agents. The Revenue's district valuer will readily investigate if it suspects that something strange is going on.”