I won't speak to UK weather. Let's just assume it's 100% earthquakes and hurricanes. (it's not, and it's ok for kids to get wet, but whatever hurricanes and earthquakes it is)
First, lets talk about the tv shaped elephant. If you don't want him to watch TV, then don't let him. If that means turning it off, turn it off. If that means unplugging it, unplug it. If that means smashing it with a hammer, then smashy smashy. You are the parent. If this is where you chose to draw the line. Be clear, direct, and follow through.
Now, you have acceptable activities. With the TV out of the way, present them. If your son doesn't want to do any of them, inform him he can sit on the floor and mope. Anything, even reading, is better than nothing. So if the choices are to sit on the floor and stare at the wall or read a book, the book will win.
Now your job, other then turning the TV off is to make sure that activities are available.
Get some crayons, a book or two, some coloring pages, a chalkboard, some kind of glue, maybe finger paints, spaghetti noodles, something that smells horrid, building blocks, legos, old cardboard boxes, and 3 wet rags. Plop them all down on the floor with the child and go, "meh, you figure it out". They will.
Mostly, you want to try to avoid, as much as you can, going, "But you're not playing the way I want you to." You have to a little bit, least the spaghetti noodles end of the wrong side of the pet dog, or the finger paints turn your ceiling the wrong color. But if it's safe let him go to town.
Looking at books is the first step in reading books. You might find more interesting books. That can be hard at first, so start with lots of books and refine them as time goes on. Anything this side of a playboy should be just fine for an 8-year-old. Context might need to be given for some more complicated books, but if your son starts reading War and Piece, then let it go.
All joking aside, "I'm bored" is relative. Right now, you don't know what his interests are, so throw everything you can at him. Don't spend tons of money, but the basics are pretty cheap. Once an interest is expressed, you can worry about how you're going to fund it. Turning off the TV, by whatever means necessary is step one.