I have carried out mock interviews with sixth formers and recorded on to VHS tape. We powered the camera from the mains but, if you must have batteries, you can buy long life batteries from third party suppliers.
Are you saying that the interviews will last 2 1/2 hours? Normally an hour is enough. In which case change the battery between interviews.
As for your £100 budget you will be hard pushed to find a new camera. Second hand may be an option. Do not go for a DSLR as they tend to overheat with long use.
I work for a company that needs to record lots of mock job interviews. We are looking for cheap cameras (£100 is the absolute maximum) with the longest battery life. 2 hours is the minimum but ideally around 2.5/3 would be great.
We do not care about the quality of the footage it is just important that the cameras can be relied upon for film for as long as possible without running out of batteries.
I'd appreciate any tips anyone can give me.