Question

"میخواهم برم لندن گردی" به انگلیسی چی میشه؟

Answer 1

go on a London tour/ go on a tour of London

Comments

I'm not sure if 'go sightseeing in London' works in your context. check the definition you might have been looking for that.

http://dictionary.cambridge.org/dictionary/english/sightseeing

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It works too but , the ones given in your answer post convey the meaning better. +1