How much am I likely to have to pay altogether?
This is not a particularly cheap holiday. You're going to have to pay for a lot of petrol (it's a bit cheaper in France). Assume a total distance of about 1500 miles. If you budget for about £250 you should have plenty.
You're also going to have to pay hotel bills for 6 nights. Hotels are also cheaper in France. Double rooms tend to start at around £40 per night for two people. So that's a minimum of £240.
The hotels we will be booking tend to be the more characterful ones. These often have a variety of room rates and you may want to decide whether you want the most expensive rooms or the cheaper ones. You may also not be attending the whole event.
Meals and drinks will all be extra.
We're also looking at camping options for those that want to save a bit more cash.
So there is no fixed cost but the minimum is around £600 minimum when you include the Channel crossing. That is for two people. You'll spend a lot more on food and drinks though.