I think you'll find Adrian is right and you need to find the other fees as well.
But generally the £2500 HLC is adding over £100 pm to your repayments over two years.
It doesn't look like that because you have spread the repayments over 35 years,so instead it is increasing the outstanding capital as you have twigged.(most people never spot that they just look at monthly repayment)
Whilst HLC's are a bit of a con, they do allow lenders to lend more money than they usually would.
So if you could only afford £844 pm and not £870 you would be grateful that Natwest would lend you the money, irrespective of the future cost.
You should remember that your house is security for your loan. If you default on repayment and arrears build up the chances are the mortgage company is going to get back less after auctioning the property than you owe them. At which point it makes them look pretty stupid having taken the security of an asset thats now worth less than the loan.
In all other business deals banks offer loans of between 65 - 80% of the value of the security. If they did that with houses very few people would own houses now. ( But they would be cheaper, so it is the banks fault after all!!)