You can register with MyPropertySpy.co.uk, it is free and it gives you land registry data from the last 6 years, so the properties sold in your area within the last 6 years will show the price they sold for.
Of course, your house might be worth more if it has been improved/extended etc.
There are other sites like this, that will give you prices houses sold for, you have to register with them but it is free to register.
If you feel it is deceitful to get an estate agent to come round and value the property when you know your are not going to sell, you could come clean and ask if they will value it for you if you paid them for their time. Don't know how much it would be likely to cost, but it might be worth enquiring.
Personally, I think estate agents charge such a colossal amount for selling your property, factors such as people having valuations when they aren't going to move are all taken into consideration when the estate agent sets its fees (I bet there are quite a few people who get valuations, for reasons other than because they want to sell) but I know what you mean about it feeling wrong.
I came clean with the estate agent who valued my old house - I told him we did not want to move at that time, and being the nice man that he was, he agreed to come round and value it anyway, free of charge. We did live in a very small village though, not a big town where the estate agents are probably much busier and more competitive.