How much is the average mortgage in the UK?
From recent press publications, the average mortgage in the UK is £130,000.
Keep in mind however that it's the average based on prices in the whole of UK and that includes both very cheap and very expensive areas. To give you an idea of just how massive the price gap is: average cost of 2 bedroom flats in central London is £638k while 2 bed flats in Liverpool can be purchased for (again, on average) £62k.
Than again it doesn't really answer your question since you've asked about how much is the mortgage.
Considering that the cost of property is really £130,000 along with interest rates at about 4,3% and the standard 15-20% initial payment you would pay 400£ / month for interest only mortgage and £550 / month for a repayment mortgage. This calculation assumes repayment spread over 30 years.
It's really hard to give an exact figure of the average mortgage repayments in UK because of how rapidly house prices and interest rates tend to change on the market. You can, however, expect to pay this much for your first property.