If you're looking for an early season Tri to get your session started, maybe a super sprint before you try a sprint later in the year, or a warm up race ahead of your goal races, then Bicester tri could be the race for you. 

There's still places, not sure when it will close (and guess there's a risk all the late entries will be grouped together for the swim), it's got a sprint or super sprint, easy to get to on the train and sprint doesn't start til 10, super sprosp is later. I'm planning on getting the train out and then cycling at least part of the way home, let me know if you're interested. 

https://www.britishtriathlon.org/events/bicester-triathlon-2018_9176