ukmodelshops.co.uk - is the go-to place to find model railway events.
You can search by looking over the list in date order here as well as filtering them to a set distance from a postcode here (very useful for finding local events.
If you know of an event not covered you can submit events to the site here
They also have a directory of model shops which is very useful for not only finding local shops but also finding shops when travelling.