Not an easy one as trout fishers are the opposite of what would appeal on a"dating" site. We are generally only comfortable with those we know, we can be competitive and sometime smelly.
Seriously, check out The Walthamstow Fly Fishers website and that will give you a chance to meet fellow flyfishers, and perhaps build up contacts from there. Good flyfishing available on three reasonably well stocked reservoirs
What JoeOh said or/and join a fly tying group like the fly dressers guild, closests one to you is www.chilternflytying.co.uk to meet 'like minded' people and see if you get along. They also do fishing days out.
Do give walthamstow a visit or two,the regular rods are a nice bunch of guys.
The club fee is £10,and they do odd trips to some of the other fisheries.
A good way to get into it would be join in the matches,plenty of the knowledgable anglers do those.