Hi Lucasa
I just had a look at the Standing Agreements for EGTC traffic. There don’t seem to be any out of S27.
EGTC is just north of EGGW. So offer it down to COWLY at 200. As all the LTMA traffic is FL200, that would be a safe option.
If you are not familiar with the location of an Aerodrome, use the Direction finding tool to see where it is. And make something up and see how it works out. Feel free to ask here what others would do.
Cheers, C