I live near Brussels. There was a 10K race the week before Christmas and 25 runners from Blackheath & Bromley turned up and took away a few prizes. Great race underneath the Atomium monument.
The Brussels Marathon and Half Marathon are in the first week or so of October, are easy to get into, very keenly priced and there's approx. 3,000 in the full mara and about 7,500 in the HM. Brussels is just two hours drive from Calais (motorways the whole way) plus we have fantastic beer and cuisine over here. Or 2h20m from London on the Eurostar.
The city also copes very well with people who can't speak French or Dutch but are stuck on English, which might be an issue if you go a bit deeper into the continent.