Buy crumble mix from Tesco's. Peel and slice apples (whatever you like) and bung in dish. Wash blackberries, bung on top. Sprinkle sugar to taste. Empty packet of crumble over the top and bung in the oven for 30 mins. Eat with custard.
Either rub butter into flour (messy but soothing, use your fingertips, kids like to do this) or whack butter and flour into food processor and blitz until it looks like breadcrumbs. Then stir in the sugar using a fork. If you're feeling showy offy and have them, a spoonful oats will add a nice texture, so will chopped (unsalted) nuts.
Bramleys and blackberries wont need extra cooking, just peel and slice the apples and put them in a baking dish with the blackberries and sprinkle some sugar over the top. Then put the crumble mix on top of that and stick it in the oven, about 190C for about 40mins.
Don't stress about quantities, if you like lots of crumble to fruit make more of the crumble, but do try to use about half the weight of butter to flour.