No need to worry - I've done the last two HIMUK races in Llanberis - these were hard-water temp 12 degrees brrrrr! and wind and rain along with the "undulating" terrain.
The course in Sherbourne will be a breeze -apparently it's flat, so will be good for a fast time.
I'm really dissapointed that the race didn't go full distance this year, but I have still entered.
You will have to get used to running off the bike, and cycling from the swim.
For this type of event I will do long swim sessions of up to 2800 metres and bike /run sessions of up to 50 bike followed immediately by 90 mins running.
Get used to SIS gels and bars - these are what's available on the race (have been for the last 2 years).
It may also pay to get some open water swimming practice in (around Easter time when the water warms up!)
I'm doing another full IM next April, so it will be nice to do a (relatively)shorter event.
Paul.