Having been through all this for the past 18months with DS (he has an offer and we are now on tenterhooks for results day...) I feel like I could write my own guide, althoguh Pushy Mothers' guide is excellent if getting a tad out-of-date - there's volumes on info online now anyway. For starters head to www.thestudentroom.co.uk/, scan down to find the forum for medicine - just about everything you need to know is there.
For work exp her age will be a limiting factor, so work around that. Are you N,S E West? Start off with your schools' career counsellor, ask about volunteering options for her age group. Contact local hospitals, Hospice, Care homes, special need schools. Utilise whatever contacts you have to get some volunteer ward hospital ward work or anything in a care setting.
As well as volunteer work Med schools are looking for rounded individuals, so consider DofE or similar, working in the community, sports groups where a leadership role might be involved as wel as demonstrating good communucation and interaction skills. Add to that personal skills like organising a fund-raisning event within her school/church, mentoring younger students, even some sort of young enterprise involvement.
Once GCSE results are in, start scanning through the various med schools entry requirements - many have a minimum A entry (ie Birmingham) and may specify what those A must be in, so tailor med school selection from there. Then read in depth what those schools are looking for in candidates and work towards those targets.
For early next year there are free 'taster' days for medicine offered in London (dont' know about elsewhere) - these are booked incredibly quickly so keep an eye out!