It's good to see members from UK join the forum. I use WhatCar? magazine's website quite a lot to see what kind of vehicles the UK has access to and what we do not (unfortunately we woud like to see more smaller displacement or Diesel vehicles). I look forward to your insight on cars and driving in the UK.
So, tell us more about your Nissan...it looks like the Rasheen is a small SUV with a 4-cylinder petrol engine? Getting 30mpg-Imperial isn't bad for an SUV -- good work! I think with your platform, there is definitely some potential.
What kind of driving do you do mostly -- high-speed Motorway or more 2-lane "A" or "B" roads? Lower speeds and some modifications may be a good way to make improvements.
Foremostly, welcome, and please let us know if you have any additional questions, as we are always here to help.