I don't know what the import duty would be (U.S. company) but look at Cabela's online. They have a line of expedition dome tents suitable for a family in MANY sizes. Their tents are bombproof and can be returned for a full refund if there is ever a quality problem.
In the past I have usually always factored in about 5% on top of vat for tax and duty when importing stuff. It doesn't always work out anymore especially if its really low priced item but its over the threshold of £18.
Royal Mail have this dreadful handling fee of £8 on all standard mail items coming into the country. I call it a blackmail fee because you have already paid postage, and you have to pay an additional charge to get your item which is often delayed by them. {I have an ongoing dispute with them at the moment over an item that got delayed - their own postal services act says they can't delay the mail for fees lien payable "i.e. blackmail"}.
If the item is send with standard airmail and not an express postage service then the cost is going to be whatever the total price is in us dollars (declared on the package)plus the cost of the postage displayed on the package, add 17.5% on top of that for tax, add £8 royal mail extortion fee, and possible a small percentage duty (duty not always payable on certain items) then you have the true cost.
A shame the exchange rate to the dollar has dropped recently down with dollar strengthening a bit from app 1.66 just a few months ago to 1.51 (has been slightly lower 1.49 a week or two ago).
If your item is coming by express mail service the fee can be a lot higher than £8 it can vary as a percentage. That's if its handled by Royal Mail at this end. If it comes via UPS, DHL, Amtrak, Fed Ex, they can have their own set fees which may be better if you know in advance.
John