Go to as many free or cheap local events as possible. Take picnics or eat at home, food out is generally overpriced, poor quality and at the end of a long queue. Or possibly compromise with supermarket meal deals or their cafes often do kids eat free type deals.
Look to see where you can go cheaply on public transport. From Hertfordshire I would have thought you could get to London quite cheaply so go to the museums, big parks, just wander around for the day. If you go on the train, places like London Zoo do 2 for 1 entry, its a National Rail scheme so look into that.
You're far from the beach but there will probably be (organised) lakes where you can swim and spend the day for relatively low cost.
Do you have anything you can sell to raise money? My village is running a table top sale in early July, good for a clear out and bring some money in.
Unless your credit rating is awful, you could change your bank account a couple of times in the next month or so and possibly bring in £3/400.
Review your budget. With 3 DC and no benefits, your income is probably decent so could it be that reducing your spending in other areas will free up money for school holiday entertainment?
Could you stretch to a night or two in a youth hostel or Travelodge? Many are in coastal areas.
Also look at Moneysavingexpert.com and Days out with the kids. Both have big features on how to entertain DC cheaply in the holidays.