Given that both the -ize and -ise spellings are both OK for British english I don't really think that this is the problem.
The problems I see are:
1. Everything Is Capitalised And This Is Not The Usual Way We Do Things In English Is It? Capital letters go at the start of sentences and start proper nouns. Personally, being a picky proof-reader, I would change all the wording to the correct case.
QUOTE
\"The door was the way to... to...The Door was The Way. Good. Capital Letters were always the best way of dealing with things you didn't have a good answer to.\"
(Dirk Gently's Holistic Detective Agency, Douglas Adams).
2. There are some phrases that are Americanisms and we have Britishisms to match (see a fanstatic book called "British English A to Zed"). Thus, I would make:
#5 Mute (the word "microphone" is redundant)
#6 Put call on hold
#7 Engaged
#52 Call length
#65 You cannot call yourself
#70 End call
#181 County
#242 Date of birth (we use this phrase as it used on Birth Certificates, and is also abbreviated to DoB)
#246 County
#247 Town or city (cities in the UK have that status conferred by the crown, not just because it's a big town. Tiny Wells is a city; huge Blackpool, Colchester, Croydon, Doncaster, Dover, Guildford, Ipswich, Luton, Maidstone, Medway, Middlesbrough, Milton Keynes, Northampton, Reading, Shrewsbury and Atcham, Southend-on-Sea, Stockport, Swindon, Telford, Warrington and Wirral are not)
#325 Send engaged tone to %s
#399 Date of birth
#449 End call
#459 End call
#467 County
#468 Town or city
#547 All calls will be ended. Continue?
#637 Date of birth
#668 Mute
#669 Remove mute
#872 Exit Skype?
#873 Exit Skype?
#901 Call is on hold
#902 Call is on hold
#1007 The Skype Name you entered is not valid!
The word "quit" is British english is used when people leave a job, it is not used as a synomym for "exit". British english speakers don't clearly see the subtle difference between "Quit" "Cancel" "Stop" "Exit" and "Close".
We also call pronounce the zero digit in telephone numbers as "oh", and we use the "n-double-n" rhymic format rather than "tripple-n". So (020) 7446 0002 is read:
oh-two-oh seven double-four six oh-double-oh two.
We call busy "engaged" and it's a "tone" not a signal. Signals are the things that direct railway trains (not cars, those are traffic lights).
Finally, I would replace the words "Instant Message" with "Text message". All UK mobile telephone handsets (which 90%+ of the population have, including kids) have SMS text messageing, and millions of messages are sent each day this way. If you are going to use the telephone metaphor for Skybe, the words "Text message" would be more suitable for the British.
Excellent program, Skybe, BTW