(AB's server is only accepting very short posts, so I'll split this up):
'Registered Post', as such, no longer exists. (The service has been absorbed into 'Special Delivery').
Use Recorded Delivery if you just want to avoid any possible arguments with the purchaser over whether they actually received the tickets. This just costs 68p more than normal postage (either 1st or 2nd class). So you can send the tickets, 1st Class, for just �1.00. Note, however, that if the Post Office lose your mail, you'll have to refund the purchaser but the Post Office will only offer you minimal compensation. (Typically half a dozen 1st Class stamps).