I reckon they will have used a Welsh rarebit style topping. You need to get hold of some pale ale, something like Oxford Gold or Spifire so that the flavour is subtle, it certainly doesn't have a beery taste.
http://www.bbcgoodfood.com/recipes/3660/the-cheese-society-welsh-rarebit
Instead of following this exactly though I'd make the roux with a good bit of butter and only a teaspoon of flour and cut down the milk a bit too. Adjust the seasoning to your liking, if you want it more cheesy then cut down the worstershire sauce and use dijon instead of Colmans.
http://www.bbcgoodfood.com/recipes/3660/the-cheese-society-welsh-rarebit
Instead of following this exactly though I'd make the roux with a good bit of butter and only a teaspoon of flour and cut down the milk a bit too. Adjust the seasoning to your liking, if you want it more cheesy then cut down the worstershire sauce and use dijon instead of Colmans.