Hi Ciaran,
I'll stick my neck out a little here. You reside in the USA, I'll presume your 'life' is now set there and you don't have any financial ties to the UK? be that in bank accounts and tax domicile, savings etc? (I know you say you have a current mortgage in N.I)
In that case you'd be a 'foreign' investor, and I'd have thought with a big enough downpayment/deposit you could potentially secure this mortgage. When money was being thrown around like confetti a few years ago, 'foriegn' investors could put down perhaps typically 25% downpayment and they'd pretty much get mortgaged for the rest with minimal credit history etc etc. Of course, now after the bubble burst - it's still possible to get the same, however I'd estimate you'd need to show 50%+ downpayment and perhaps some more proof of income etc. I know you say you are re-mortgaging, so you'll be assessed on the amount you want to borrow.
Only you can assess your personal scenario, your income levels etc to see if you'd be able to get re-assessed.
Lending here is all but stopped, and to get the best rates, like this mortgage seems to be currently, you have to be squeeky clean on UK credit history and have a very large deposit. As far as I'm aware - the HSBC premier account listed on one of the links, requires you to have $100,000 in balances in the account.
finally, be very aware.. as thats a tracker (which is the mortgage I have - I pay a Base Rate +1% rate) and the fact interest rates are ONLY going to go up over the next few years, ENSURE you factor that into any repayments. Call it 'early' scaremongering if you like, but some current reports say interest rates could rise as high as 8-9%, by adding on your tracker rate - do your calculations now at potentially 10.6% or slightly more!
Good luck!
DtM! West London & Slough UK!