You are putting deposit for base spec helicopter (which seems to be nice enough as it is) with options/options pricing
TBD, so basically Hill can charge whatever he wants for options. You as a buyer have 3 choices: go with base spec, pay whatever is asked if there is something you really want/need or cancel order forfeiting your deposit.
I don't know how it is defined by UK law, but in my country, overdue payment penalty interest is defined by central bank. Hill charges 4% on top of that.
If customer refuses to take delivery (doesn't state what is reasonable refusal), buyer is due full price+plus any storage expenses.
Some other things, but I am not versed enought in legal contracts to be able to fully understand implications.