Although I'm based in Canada, I'm sure that you can find trainers in London (England I presume) by searching Filemaker's website for trainers based in London. Although given what you are looking to accomplish, you may want to consider a developer who's willing to train you instead. It will be less formal than classes but you are more likely to get what you are looking for at the end (a working database that is matching your requirements). Trainers usually have preset training material.
How much? Well that depends on how complex your solution is going to be and how fast you can learn and how much of the work you will do yourself. They will most likely bill you by the hour so $$$. Have you considered purchasing an off the shelf solution instead?
Filemaker Pro can technically host your site but as you noticed it is limited to Instant Web Publishing and can only host a maximum of 5 concurent users. Since IWP maintains open sessions, you can quickly reach that limit (users often forget to logout). Unless you have significant web expertise, you may want to look into using a Filemaker Web Hosting service instead (there are several to chose from).
Est-ce que ça répond tes questions?