Surely sales tax depends on which State you are in. I assume all States have some Apple stores, and some, such as New Hampshire, have no Sales tax, so the difference can range from 90-ish pounds for the cheapest model, to over 130 pounds for the most expensive.
I'm not advocating the purchase, and doubt it would arrive completely unqueried by customs, unless you were visiting friends, got them to buy it in advance of arrival in NH, opened it while you were there, and saved a few documents etc on it. Gave friends a nice gift and cash to cover sending it to UK, and then they could wrap it as a "used, but fairly new" birthday gift for their godchild... perhaps.
It would not look pristine as it had been opened. You could bring back accessories, so it would be bare unit in bubble wrap, and lack that 'brand new' look. Never know, it might not be that common a happening, and even with an approx value of half the cost (as a second hand unit) of the basic model, on any customs declaration they fill in, it's difficult to guess what it would have cost without knowing the model... and whether the sender got a big discount as an employee in Apple Store, or wherever.
I don't suppose the battery can be removed easily though (which could make it more difficult for Customs to check it).