Hmm, I think this could be fraught with problems.
At the very least the same Notaire fees would apply as for a sale. Also, it being your only residence does not of itself exonerate you from CGT.
If it is a new property for instance or if you have owned it for under 5 years a liability could arise.
If there is a mortgage on the Welsh property then it has to be paid off before it can be disposed of, be that by swap, gift, or sale, you cannot assume responsibility for it.
PS: Chrissie, your site seems to be for holiday house swaps