It has never been easier to buy tickets than it is today. Most theatre Box Offices now offer a 24hr/7day telephone credit card service, so you can call from anywhere in the world at a cheap rate. You can charge tickets to your card, and if there is time they will be posted to you, if not you can collect them at the theatre on the night. Do arrive in plenty of time as there may be a queue, and you will need to bring the credit card with you and sign a receipt. Some theatres may charge a service fee for using a card but will advise you of this at the time of the booking.
Most ticket agents, and resident companies such as the Barbican and Royal National Theatre, will now accept credit card bookings by fax. You will receive a faxed confirmation of your booking, and the tickets will be posted out or held for collection.