I'd agree with Sarah and italicise the entire thing. After all, the
word is "London's" - the apostrophe and s are part of it.
If you only italicised part I'd be thinking there was some special emphasis on London or the s, such as you might use to indicate a particular usage, e.g.
"I don't want to use postpay, I want to use prepay" |
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However, like Sarah, that's just my opinion, I've no idea if there's any firm guidance on it.
Jon