I'll give my backing to Mushroom25's route. If you do decide to travel solely by train you can save a great deal of money by booking through tickets from your local station in Yorkshire to Calais International on the Eurostar website. (The site doesn't allow you to book to Boulogne-sur-Mer). I've just checked some random dates in October (out on 6th, back on 13th), choosing Leeds as a departure point. You could leave Leeds at 1640, arriving in Calais International at 2159. The return journey would mean leaving Calais International at 0934, to arrive in Leeds at 1254. The return fare is just £85:
http://www.eurostar.com
However that route means that you'd have to get a taxi (or possibly a bus) between Calais International and Calais Ville (which is where the trains to Boulogne-sur-Mer run from). From memory, I believe that those trains are rather infrequent. (Use Mushroom25's link to check).
Your alternative would be to take a train to Kings Cross or St Pancras (which are next door to each other). Then take a direct train from St Pancras to Dover Priory:
http://www.nationalrail.co.uk/
From there take a taxi (or bus) to Dover Ferry Port for the LD Lines service to Boulogne:
http://www.ldlines.co.uk
Chris