I always say to people that it's not as precise and exact as that - a sort of 'tick the boxes and you're in' kind of thing... I encourage study of the doctrines so that a person can see if he or she agrees with Orthodox belief. Assuming there is such agreement and the person desires to enter the Church then I would receive them as a catechumen during which time they immerse themselves more into the spiritual life of the Church, praying the daily prayers, attending the Liturgy, celebrating the feasts and participating also in the fasts... this period can be short or long depending on the person, for some it is a matter of weeks or maybe months though for some people it can take longer. At any rate there should come a time when both catechumen and priest agree the catechumen is ready to be baptised and chrismated Orthodox.
Not that learning stops with baptism as we hopefully all continue learning more and more of the Faith throughout our lives - neither am I suggesting prayers and worship cannot be started during an initial enquiry stage prior to becoming a catechumen as this is a living relationship rather than something bureaucratic. Since you have been considering things for a while now anyway and desire to become Orthodox I would hope you could be received as a catechumen without any great delay.
As for being baptised in London during a visit, might it not be better to be baptised and chrismated where you are (or as near to you as possible) and where you will presumably worship?