I know that BA have strict requirements on measurements and fitness tests for there pilots because their seats only take certain sizes. I think there max height was 5'11" but I could be wrong. Also, 18 stone might be a little on the large size for there seats. But when you say big, it sounds like you mean well built so there probably isn't much you can do about it. If on the other hand you are overweight and you know it, then start to lose it now because if you don't, you might not even get the class 1 medical. Remember that class 2 medical you got? That is the tip of the iceburg to the £400+ initial class 1 medical you have to get before you even think about embarking on a commercial course.
Also, if you have money on you side then go for it because it is VERY costly. I am 3/4 way through the ground school and it has so far taken me nearly a year to complete. I don't know of anyone who has passed all 14 ground subjects first attempt, so if you are like me when I first started thinking, 'well I'm bright, this should be a breeze,'think again. It is hard, and many have likened it to a 4 year degree in 8 months. You have a good chance if you are a hard worker, commited and ready to make sacrifices of the social kind for the next year. Aviation isn't for people who just like flying. You have to really want it and then some more.
Best of luck, and hope to see you in that left seat!