As well as the tax duty, you're very likely to have to pay the "ransom fee" to Royal Mail/Parcel Force depending on who has your parcel and what speed it's coming. The last time I was ransomed by Parcel Force for express delivery (on a parcel from Korea marked $50) I had to pay £13.50 to PF as a handling fee on top of the customs charge, bringing it to almost £30. Their regular fee is £8 for non-express, and I think Royal Mail is a little under that.
You should get a letter from PF/RM saying you have a parcel waiting and you need to pay X amount, from my experience. You can then either go to your nearest PF depot or RM sorting office, pay your fee (do check with RM though, as my local office will not take debit/credit cards and I had to pay in cash, although this was a couple of years back). PF will also let you pay online and schedule a day for delivery of your parcel, it depends on how far away your depot is and how fast you want your ponies!
I'm afraid I don't have any experience of parcels coming from abroad through UPS, Fedex or any other courier where I got charged customs so I can't help with those.
I'm so sorry you got caught! It's pot luck, and I usually get caught for every US parcel I have. T__T I hope you get your ponies really soon. <3