VLM haven't as far as I am aware advertised in the local press nor have I personally seen any advertisements regarding their SOU routes in any Shipping Journals or company emails, the only place I have seen them advertised is at SOU airport as well as in the Airports bi monthly magazine.
During the summer months especially there is quite a bit of traffic to and from Hamburg and Southampton and most of that traffic goes through either LGW with EZY and BA and LH through LHR with a wider schedule.