The Government changed the VAT rules in the June 2010 budget so that some Royal Mail services are now subject to VAT. (The change came into effect on 31st January).
The following services now have VAT added:
Airsure to EU destinations
Special delivery by 9am
Special Delivery by 1pm (but only when bought with Saturday Guarantee and/or Consequential Loss)
Standard Parcel (but only when bough with Additional Compensation)
International Redirections to the EU
Local Collect
Keepsafe
Prepaid stationery for Special Delivery by 9am or the Airsure (EU) Air Pack.
All other postal services remain free of VAT (other than as below).
You should definitely have been charged at least 88p in VAT (20% of £4.38) but whether the postage was VAT-able might depend upon which service was used (but, again, see below).
However (entirely separate to the budget changes) the VAT rules have always included a 'follow through' provision in relation to carriage charges, so that the carriage charges will normally be taxed at the same rate as the goods which are being supplied. So if you buy books (on which there is no VAT) you shouldn't be charged VAT on the carriage, but if you buy VAT-able goods you should be charged VAT on the carriage costs:
http://tinyurl.com/4zt65m8
Chris