The question I would ask you is how many frames of brood do you have in your brood box now? Colonies will be starting to slow down their brooding now as we are past the longest day so the brood nest us unlikely to increase much. Note that 1 frame of brood gives 3 frames of bees, so they may need a super now just to give the bees room. If you are hoping for some honey over the next few weeks, then a super is the way to go. Once the honey is off - then you can use a super for winter stores along with the brood box.
Read this for winter preparation tips.
I am in the southern tip of Scotland, over 300 miles south of you, so my weather is different to you so the bees will behave differently, however you should anticipate that Buckfast bees will need more than a National brood box in a full year.