I agree with Dave. Keswick is the best place. Windermere is easy to get to because of the train, but it's not very photogenic and quite touristy. Ambleside is another option but again it's not that photogenic and is mainly a base for walkers. I'd catch the train up to Penrith and then the bus to Keswick. Sounds like a lot of your time will be spent travelling so you'll have to get up early According to www.thetrainline.com you could leave Euston at 06:46 and be in Keswick by 11:05. Although I'd be surprised if it's that quick!

Mike