Hi Falabo,
If you mean conveyancing the properties (changing the ownership) then you could not keep your own mortgages. Stamp duty would be chargeable on both properties at open market value.
If you were talking about renting each other's properties then you would both register with prtb, pay nppr tax, lose trs, pay income tax on rent receivable less deductions, possibly lose tracker mortgages.
That said, plenty of people have had to move to larger properties, etc, and become "reluctant landlords" but I don't see any legal/financial advantages to swapping with a friend. If anything, if one of you ends up late with rent or maintenance then you could end up falling out badly.
Sybil