Europe (London/Paris) in May 2019 - when to start booking?

Feb 2019.

You won't even be able to book trains until @ least 3months ahead.

Flights schedules come out approx. 11 months early. Prices are cheaper the earlier you book.

This also applies with hotels. Book a cancellable hotel rate early, and then keep checking if rates get better.

Eurostar - keep an eye out for seat sales as you get closer to your date.

We did London/Paris this past summer and managed some great rates by booking early.

We did a tour of Morroco last Sept and Oct. We Did AB&B, and bullet trains from Spain to London stopping @ various places along the way.
Booked the plane in April, the trains and accommodation a month before we left.

Going to London tomorow, and I booked that flightFeb 1, and hotel about a week ago.

Start checking prices and get facebook alerts from nextdeparture and secret flying now already so you'll have a feel for what a good deal for flights is, even though you won't find any flights for May next year until a year before. Most likely the good deals will start coming around October. A month or two before the dates the prices will just keep going up most likely. All dependant though on how flexible your schedule is. Also look for flights into Brussels or possibly Amsterdam if you don't mind visiting another city and doing some train travel (cheap if booked a month or two in advance). Hotels, book far in advance, refundable. And a little closer to the date see if you find something better and cancel your old reservation if you find something better.

Go to flipkey and look at private rentals. Last time I was in london I got a flat for 4 days with a kitchenette basically right across from hyde park for the same price as a tiny hotel room was in a slightly less posh area. Gives you a good base of operations and much more private with the ability to cook if needed.

Book those as soon as possible. Same with the train to paris, because the prices increase the longer you wait.

I'm going to London in March 2018 , booked my flight in June 2017 as i got a deal for $300. Notified via Next departure

i did a similar trip last year to Barcelona and booked it close to 8 months before as I got a good fare through a mistake fare. Keep an eye on those

Which city first ?

It doesn’t really matter... best strategy is to figure it out by doing your flight research... 2 weeks apart & 2 cities (flying into one & out of another) CAN MAKE A DIFFERENCE when it comes to pricing.

Prices differences can be substantial.

Also...
Remember you get better Consumer Protection if you fly both ways with a European Carrier... or in the very least Europe to NA if you use a Cdn Airline. So be OPEN MINDED and just don’t automatically go for the cheapest airfare (example: personally I would NEVER fly AirTransat cause of this... as clearly it’s been shown this year they are having Customer Service issues).