Hi Robert,
I live near Oxford and work in Rotterdam, so I drive this route at least one week in two - and equally I don't have any experience of the Rosyth / Zeebrugge ferry.
Zeebrugge in Belgium is about 1 hour 30 minutes from Calais, in a North Easterly direction - just along the coast.
Zeebrugge is only 30 minutes from Brugge / Bruges - and it's a great city for a few days - good food, fantastic beer, really pretty city centre and city walls. Well worth a two or three day visit.
Dover - Calais has the advantage of frequent crossings - if you miss or arrive early for a boat or train, then it's usually not a problem.
You need to weigh up the costs: The Dover-Calais crossing and the fuel (and other costs) in driving down the UK to Dover, and compare that to the cost of the longer ferry crossing.
Also if anyone in your party has a problem with sea-sickness, then the shorter crossing is probably better, but I've even seen people looking unwell on the EuroTunnel.
Tip: If you decide on the longer crossing and stay near Brugge, you may want to stick with English until you are over the French border. This Northern part of Belgium speaks Dutch (Vlaams) and you won't be too popular speaking French.
Tip: If you decide on the longer crossing - then pick up fresh food in the massive Auchan Hypermarket at Grande Synthe (Exit 25 on the A16 / E40). A lot of Belgian people shop here as it's so close to the border and prices are better than in Belgium. The A16 motorway runs from Brugge / Zeebrugge past Dunkerque (and Grande Synthe) and on to Calais.
Regards, Robert