Ideas
You could ride to West Hampstead ThamesLink Station, about 1 mile from Kilburn High Road. About 40 mins to East Croydon on a train, check at the trainline.com for times and bike restrictions on this particular route. This route could also serve as a return route from the City at journey's end.
OR a longer ride down the A5, Kilburn High Road, Park Lane, etc. towards Victoria Station, then only a 15 minute express train journey.
OR a similar ride down towards the river at Westminster, for a Thames Clipper service to Greenwich (quite an adventure as they are very fast catamaran ferries which take bikes, plus some great views of London landmarks from the Thames too).
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I will probably go for the black Muirwoods, 26" wheels, 2" tyres, rigid frame. It is a bit slower than my 700c bike, but very comfortable and quicker to change inner tubes if punctured.