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Something I was pondering earlier:

Generally speaking, a "Not via London" ticket means "Not via a London terminal"/"Not via Zone 1".

So travelling from Reading to Brighton with a change at Clapham Junction is valid, but (say) travelling from Reading to Chatham by changing between Waterloo and Waterloo East would not.

It struck me that going from Reading to Gerrard's Cross/High Wycombe could be done by changing at Hayes & Harlington, West Ealing, Greenford (for Central Line) and South Ruislip, or even Ealing Broadway > North Acton > South Ruislip, neither of which involve entering Zone 1.

Does anyone know if either of these are valid "Not London" journeys?

I think the only standard tickets available between Reading and Gerrard’s Cross have ‘via London’ on the ticket. But they still let you travel via South Greenford. (Perhaps technically you should go all the way to Paddington first, then return to West Ealing, but I don’t think anyone would query that!)

You could consult the National Routing Guide if you want. An interesting look if you've never seen it before, but is is quite a complex set of documents. It was on the ATOC website, so presumably now on the RDG one.

If the ticket doesn't have the Maltese Cross on it, then it is not valid on the underground and so there would be no way to get from Greenford to South Ruislip. (In practice you wouldn't be passing through any barriers, so the chances of anyone noticing are very low).