Its tricky - you can get so many good deals to go abroad - it can be cheaper than staying over here. I guess the fairest way to do it is to include all the add on costs, like getting to the airport, parking, transfers.
It depends where you go it can be cheaper to eat (and more importantly drink!) when you're aboard.
For me there is places in this country, like Cornwall, that are absolutely stunning. On balance - go for what you can afford! I'd say it pays to have a shop around and do some internet research. Finally, its not aways last minute dot com - I've got some really good deals by booking early (particularly flights). Booking them at the New Year for the summer for example - same applies for booking parking at the airport! :-)
Not just cost, holidaying in England is fantastic, particularly when the sun shines. There's beautiful beaches, stunning countryside and fantastic city breaks. It was great this week to see how much money the tourist industry provides to the economy. My recommendation would have to be York for a city break. And for city breaks I'd recommend thetrainline.com - book early for cheap tickets and let the train take the strain!