What can/should I do to prepare for undergrad Geology? Watch
Many people do a work experience or placement that's related to their desired but I can't for the life of me figure out what I can do to show enthusiasm for Geology, outside reading books of course.
Have you looked for public lectures/podcasts/online lectures?
Where do you live in the UK? Have you looked into doing some field trips to geological areas of interest nearby (http://www.southampton.ac.uk/~imw/ is an amazing resource for south coast field trips - it looks a bit dated but the content is excellent)?
Have you looked at university websites to find out what research academics at your chosen university are working on?