The average salary in the UK is around £26,000, with the average household income around £31,000. About 90% of the UK population earns less than £40,000 per year.
In general, groceries are much cheaper in the UK, as well as things like phone/TV/broadband packages and cellphone plans... oh, and you can get second-hand cars much cheaper in the UK than in the US as well, but most other things are more expensive.
Generally, you should expect to pay the same amount or more in GBP as you would pay in USD (i.e. a $10 DVD in the US will cost around £10 in the UK).
In terms of house prices, in the US, my aunt has a large, 5-bedroom, 3.5-bath house with a pool, which is about 2,000 sq. ft. and is worth about $140,000. Last year, I bought a tiny (less than 1,000 sq. ft.) semi-detached, 3-bedroom, 1.5-bath house in one of the cheapest areas of England for just over £150,000. The same house back in my home town in south-west England would cost about £250,000.