That's a lot of money, but the norm these days I'm afraid 😬
Some tips:
At risk of sounding old-fashioned, do try to but any thoughts of 'competing' or 'keeping up' with other parents out of your mind. Do it all yourself!
As PP has said, invite no more than 6-10 children. Self-cater with the usual healthy dips, sandwiches, crisps, plus supermarket birthday cake etc.
Activities - cheap and cheerful. For a sea/ocean theme, you might want to try something like https://www.scouts.org.uk/activities/flapping-fish/
Children can make their own fishes/whales/starfish as part of the activity, play the game, and then take their creations home with them.
Then the usual, pass the parcel, musical chairs, etc. Four year olds will neither know nor care that it's a £2.50 per head tea and not a £400 event.
Party bag - cake, a hand-written 'thank you' card for the guest and, if the budget allows, a small book - these are great and under £10 for 24 - three each!
https://www.amazon.co.uk/TWIDDLERS-Pack-Colouring-Activity-Books/dp/B0BS9VYB48/
If you do want to spend a bit more on a paid-for event then just limit this to cousins and you can explain that it's 'family only'.