$100, at current exchange rates, equates to �54.35.
The duty on clothing is charged at 12%, so that's �6.52 BUT duty is waived if the amount is less than �7. So, hopefully, you won't have to pay any duty. (NOTE, however, how close to the 'cut-off' this figure is. If the CN22 customs declaration shows the value of the boots as over $107, or if HMRC are using a slightly different exchange rate, you might have to pay around �7 duty).
VAT is charged at 17.5%, so that gives a figure of �9.51. (NOTE, however, that if duty is charged, you have to pay VAT on the duty as well. Assuming the minimum �7 duty, the total VAT would then be �10.73).
On top of those figures there is the Customs examination fee of �8.
So, if your boots are judged as being just below the �7 duty threshold, the postman will ask you for about �17.50. If they're judged as being just over the threshold, he'll want about �25.70.
Chris