This might be of interest to those reading this thread. Just noticed a mention of BMI in a letter from the CAA Medical Dept.
Currently under JAA pilots with a BMI over 35 must provide a report from a CFI or Training Captain confirming that their weight does not interfere with any tasks in the cockpit environment, paying special attention to all emergency procedures
ATCO medicals are presently still under the old UK rules, not JAR, but I would think that the general thinking at LGW would be similar.