You can't avoid paying it. Your mortgage offer will state whether you can port the mortgage product. I don't know under which circumstances the porting will work. You would need to apply to RBS to accept the new property - valuation fees will apply - and even then there is no guarantee that it will be accepted. On top of this if the new mortgage is for a different amount to the old one it will cost you anyway -
If you are borrowing less then they will charge a penalty on the reduction.
If you are borrowing more then they are likely to charge you their Standard Variable Rate for the increase - around 7.5%