You'd stand more chance of responses on the Private Flying forum.
If the LARS is at the same airfield as the MATZ, then just call the LARS frequency and tell them your requirements. Although it's not mandatory to call, it's highly advisable, especally if it's an airfield operating fast jet types. If you're cleared through the MATZ, make sure you tell them if you need to vary your height (I say height because RAF use QFE rather than QNH in the MATZ) as they'll be co-ordinating you through against other traffic based on your reported height.
You should get FIS automatically, but you can ask for RIS if you like, then you'll be told about all other traffic which might conflict with you.