Hi
Another possibility is stay Camping Chateau du Gandspette.
http://www.chateau-gandspette.com/campsite-france-nord-pas-de-calais.html
There is an English speaking vet within walking distance of the campsite and it is an easy drive to the ferry at Dunkerque. We used this site and vet several times when Max our collie was alive.
We too have used the vet in St Omer arranged through Camping Chaumiere. I think the wife of the campsite owner works for the vet and will make all arrangements. St Omer is a few miles from the campsite. However I would not recommend that campsite. We found the male owner rude and difficult. We stayed both on our outward and return journey and both times found him difficut and would NEVER stay there again.
In addition Chateau du Gandspette is an interesting area with a couple of supermarkets, including one with a petrol station in the village. Camping Chaumiere is on the other hand rather remotely located in a tiny village with no shops. Chateau Gandpette is much bigger site and a pleasant one with a swimming pool, restaurant and other amenities.
I hope this helps.
Tim