How long are you going for? If only for a weekend, then you could get by with one burner. If it's for longer than that, then you need a two-burner stove.
With regard to electric hook-ups, you must buy a proper one. The ones on e-bay for £20 are not safe. The safe ones cost between £40 and £50. You could then take a toaster, but you have to be careful not to overload the electric circuit and so not use any other big drawing electrical gadget.
Only done a couple of trips but I've cooked quite happily for two adults and a child on one burner - we're going to camp more from now on, and I don't feel the need to buy a better cooker (which is just as well, as I feel the 'need' to get a lot of other stuff). I just juggle pots so everything ends up more or less hot in time to be served.
No complaints about this EHU from Go Outdoors - I wouldn't want to pay £100 for one but I agree it's not something you want to go for super-cheap.
We started with a 2 burner like this and it did us fine for years. You can always add a disposable bbq as well. Two burners means you can cook pasta and a sauce at the same time or boil the kettle as well as cook.
I now also have a Cobb which is a big investment but used for almost anything.
Re EHU agree that going cheap is risky. They can get wet and tents are a huge fire risk so not an area to economise.