I would suggest Machine Mart. They have shops, magazine and a website of the same name. (I am not affiliated with them). Prices as low as £75. B&Q also sell them. I believe you can also get an invertor to convert 12V dc from your car/van into 240V ac mains. Maplins sell them in their high st shops.
Make sure you get one rated at a higher power output than you need. If you run it at max power you may get a voltage drop or a risk of overheating.
I'm not sure it would be wise to get one second hand. It's the type of equipment that people run until it's worn out.
As a side note, if your generator is going to serve any sort of survival, backwoodsman, or civilisation breakdown purpose, I would suggest a diesel model. They are more robust and you can run them on cooking oil if fuel becomes scarce.
machinemart.co.uk, personal visits to B&Q and Maplins in the recent past