I've long been familiar with the UK spellings 'centre' and 'theatre' as compared with US 'center' and 'theater,' but I recently encountered (in a book by a British author) the spelling 'centring' for the first time. I first assumed that it was a typo for 'centering,' but I looked in my big blue British dictionary and found that the verb 'centre' does in fact have the participle 'centring' in the UK.
UK pronunciation being unrhotic, 'centre' is pronounced 'sent@,' where @ stands for the sound of 'o' in 'bacon.' The spelling 'centre' makes me want to say 'sentr@,' and that feeling is reinforced by seeing 'centring' pronounced 'sentring.' There are lots of words ending in '-er' in a British dictionary that are pronounced as they are spelled. I wonder what quirk of language evolution led to the spellings 'centre' and 'theatre' having the last two letters apparently reversed. Why 'renter' but 'centre'? If 'centre,' why not 'carpentre' instead of 'carpenter'? If 'theatre,' why not 'debatre' instead of 'debater'? Egbert White, > "I love Americans, but not when they try Planet Earth > to talk French. What a blessing it is that