Hi
I asked a similar question and was advised that it is the purpose for which the mortgage top-up is used that is important, thus, you should be able to claim interest relief on that portion of the mortgage. My q concerned releasing equity from our ppr and offsetting interest on that portion of mortgage against rental income from a UK property.
On another note, for the other 85% of the mortgage, the interest rates in UK are higher than here, which you probably know.
Regards
Folly