It's not clear whether the price quoted is in US dollars or Hong Kong dollars. It makes a difference, as I'll try to explain:
Import duty varies greatly between different electronic items. For instance, there's no duty on computers, 4.9% duty on webcams and up to 14% duty on camcorders. However, import duty is almost certainly irrelevant in your case anyway, because the duty is waived if it comes to less than �7. US$49 is about �26. HK$49 is about �3.35. In either case, even the highest rates of duty won't bring the amount up to �7, so there's no duty to pay.
The next thing to consider is VAT (at 17.5%). VAT is only charged if the value of the goods is over �18. So, if the value is HK$49, there will be no VAT to pay but if the value is US$49 then VAT will be charged at 17.5% of (about) �26. This is approximately �4.50.
Lastly, there is a fee, payable to Royal Mail, for carrying out the customs examination of the package. This fee is waived if there is no duty or VAT to pay. Otherwise it's �8.
So, if the value is HK$49, the sums add up as follows:
Duty = �0. VAT = �0. Fee = �0. Total = �0
If, however, the value is US$49, the sums become these:
Duty = �0. VAT = Approx �4.50. Fee = �8. Total = Approx �12.50.
Any item which doesn't carry a CN22 customs delaration, or where HMRC believe that the information on the declaration has been falsified, is liable to seizure by HMRC.
Chris