Well people at the end of platforms would be spotters, mainly interested in collecting train numbers, and not usually the best place to start even if that's the natural assumption. Not many people collect numbers these days as a percentage of enthusiasts.
The best port of call is one of the many heritage railways around the country, there is nearly 100.
http://www.heritagerailways.com/ukmap.html
Model railway clubs is another, also very big model shows like Warley or Alexandra palace, there are many 100s, and of course the internet, with many forums and newsgroups. There are many other clubs and societies interested in all aspects of the railway, sometimes quite specialist.
The other option is railtours, both steam and diesel. Here is a list of steam ones.
http://www.uksteam.info/tours/trs10.htm
Younger people tend to be into the computer game side TBH, for example
http://www.uktrainsim.com/