Hi Podds - my local tri club runs a DIY half day which is open to club members only and is a training day, not a race. Rough timing is available via stop watches but there are no mats, no numbers, no prizes and it's self supported. The charge for this is the lake entry fee only (we pay £4 a visit to use a particular lake). Non-members would need to fill out a day membership to the club and pay a fee of £1 to be members for the day. This is for insurance purposes.
If you're running a race, not a training day, then you are looking at something completely different.
I know I'm not FB who is a BTF official (the forumite as opposed to the social media site) but I've been involved with both and I'm happy to discuss it with you if you want to message me.