You can find this event (click on pic for more info) listed on the London Vegan (some of them will be staying overnight), Leicester Veg*n (Leicester Veggies have a mini bus going) and Surrey Meetup groups, plus Volentia, etc.
Some of us will be visiting Veggie World restaurant and shop on the way back.
London Midland trains had a 'Great Escape' all day £15 ticket offer (you need to print out and take a, limited availability, voucher to the station). See comments below for more info.
For general train times, see the National Rail Enquiries website.