Pick a number, divide by 24, add your age and then put your phone number on the end. That's as close as you'll get to working out the customs charges.
You will be liable, and it'll be roughly +10% of the value of the goods (including postage) and then 17.5% VAT on top of that, and then £20 handling fee max as well. That's only a guide, but it's not going to be far off that. Don't try and get around it by asking the sender to mark down the value of the package as then that's all you'll be insured for if it turns up damaged.