British Triathlon have an
events page where you can search by location and event type. There are a few sites such as
Entry Central who process event payments and entry. They have a similar search option. There's plenty of on and off road events around over the winter. The only duathlon I knew of in West Yorkshire has not been run for a few years which is a shame. I entered but it was cancelled that year due to low numbers. Haworth Mudman duathlon and it was a corker or a route which I've ridden a few times for fun.
If you're prepared to travel a little further south to North Notts I can recommend the
Evil Sheriff Off Road Duathlon at Sherwood Pines. I've done this a couple of times. A bit of luck with winter sun has helped make it a good day every time I've done it so far.
I hated swimming originally but eventually had lessons and joined a club to improve. Now I love it and train with a tri club and a swimming club.