OP, if your serious about this you need to think about it a little more!
You say you have 10% deposit but you've not said 10% of what value. So guessing 10% of the 250K current offer from someone else (which means you've got 25K in the bank)
If you got a mortgage of 4% (not sure if this is possible any more) you'd need to pay £750 a month with a 10% deposit. That's interest only!
If you got a mortgage at 6% this would go up to £1,125 a month with 10% deposit at that value. Again, interest only. So come the end of the 25 years you'd need to sell the house to pay of the remaining 250K debt.
As others have said, if you go a penny over the £250K cost of buying (even as a first time buyer) you have to pay £7,500 (3%).
Money Saving Expert has the tools to work it all out:
http://www.moneysavingexpert.com/mortgages/mortgage-rate-calculator
http://www.moneysavingexpert.com/mortgages/stamp-duty
Best advice is to stay under £250K as £7.5K stamp duty is a hefty chunk of the money you have saved!
P.S. Best advice I have for you is work out how much you can afford each month then see what kind of mortgages you can get at what rates. Using the calculator linked above you can then play about with values to see what your maximum spend actually is. Watch out as mortgages often have up front costs of £500 to £1000!