Grizedale Forest, in the heart of the Lakes District, is very scenic and popular with riders and walkers. There's an ample car park, an information centre, toilets, picnic area, and a café as well as a bike shop with a great range of rental bikes. It has a web site at http://www.forestry.gov.uk/grizedale.
The mountain bike trails range from intermediate to advanced. None of them are easy because this is hilly country - it's not really suitable for casual recreational riders, let alone families. Many of the trails follow gravel forestry roads, others follow old horse tracks called bridal ways, and some are purpose built single track.
The website describes five intermediate trails and three advanced trails and you can download a map from their website. The trails have good signage and are generally easy to follow.