I'd drive there - a night in a Premier Inn in Dover, to break up the journey, plus ferry crossing would be about £200 (£270 if you hotel it both ways), and fuel would be maybe £150.
Was going to suggest driving/ferry.
For flights you pay per person, for ferry you usually pay per car, so if there's going to be 7 of you, the ferry will most likely be much cheaper (went to Holland last year with me/gf/our son/my brother/his gf; cost us ~£250 in petrol & ferry, from Birmingham to Arnhem, if we'd flown it would have been more like £500 + transport at the other end).
The travel will take longer, but if budget is very tight then it's something to be considered.
I'd also suggest camping; again you tend to pay per tent (maybe with a small additional charge per person), should work out a lot cheaper than hotels.
The alternative is, why not look at renting a cottage in Wales/Cornwall? The weather is a bit hit & miss, but you're far more likely to find something decent in budget
How many people is the £700 budget actually to cover? Is that just you? You and a partner? You and a couple of kids?
How long do you want to go for?