Now, this is interesting and everyone has good points. How about this? I would advise doing the rescue and DM and instructor in the UK. Assuming the OP is from the UK. The OP states that he/ she has ten YEARS to get to the tropics as a working diver.Now, to say that cold water diving is not more challenging, well...I've done both, and the tropics is way easier, and teaching divers in cold water is more challenging. No six to eight week course can teach you to be a good divemaster in ANY location, that takes time and experience. No six to eight week course will teach you about macro spotting. You have to do it, and the OP is ten years away from being able to devote the time for that.
Compare as a potential employer:
Ten years experience as a DM and an instructor in the UK, in 12 C water with limited visibilty, keeping track of anywhere from 1 to 12 divers in 5 ft of vis, and keeping them happy while making them schlep thirty kilos of gear, and doing it well enough to keep them coming back, with the added dose of having led a few tropical dive trips
OR
I just retired, did my DM ten years ago, did a few trips for fun to the tropics.
I can train you in macro spotting, I can't give you the mental fortitude that training divers in challenging conditions gives you. The customer service you need to provide differs, yes, but it is no less demanding. As for location specific training, it assumes the ocean is static, that fauna remains the same. The sea you dive in today can and does look very different each year, let alone ten years. One of the things that sets someone apart from the rest is that experience of watching the environment change, and truly knowing it. The learning of
how to see, how to read a current, how to notice these changes, IS translatable, and very useful. Diving occasionaly, no matter where you are, cannot give you this skill.
Not bashing anyone, I've done both, and in general, going from temperate to tropical is far easier. Just my opinion.
All that being said, the OP has little interest in diving temperate, so I say Phillipines or somewhere in the Caribbean. I'm sure that Thailand produces some good dive pros, but my personal experience has been that many lack what I feel are neccesary skills.
Another idea, I've recently had the pleasure of meeting and dealing with some Hall's graduates from Florida, and I was impressed. And, no, I have no affiliation or first hand knowledge, but damn, those kids were good instructors.
Nomad
The reason for my view is that I dove in the UK for many years, before moving to Asia to work in the dive industry. I didn't learnt my 'craft' as a macro-spotting dive guide until I had completed a season diving Sipidan/Mabul. Nothing in temperate waters would have provided that experience....and customers now benefit from it.
Of course the natural enthusiasm and professionalism of the individual concerned counts for a lot.... but developing an awareness of the needs of holiday market divers is location specific. The nature of the dive industry, common practices, expectations etc vary greatly in different parts of the world.
Also, diving in Canada...with the amazing marine life you have there is great.... but it won't teach you how to best identify (for example) a pygmy seahorse...or what sort of coral will support a porcelain crab... or provide you with a working knowledge of hundreds of nudibranch names in latin.
Also...when dealing with 'large; numbers... I think this has to be viewed relatively. Some of the operations I worked at in Thailand would run 3 boats a day...each with 60+ divers aboard. The logisical and organisational effort to make this run smoothly and efficiently was incredible...and depended upon a cadre of knowledgeable and effective divemasters...who understood the system and requirements of that operation.
I doubt that there are any dive operations in Canada that provide diving services for 180 divers, twice a day, 7 days a week......
Add to that the 'holiday diver' mentality - with people needing rental gear, not knowing their weighting requirement, turning up late for diving, not remembering how to assemble their gear...and generally wandering around aimlessly unless effectively directed by the dive staff. PLUS, the fact that they all have very high expectations and demand personal attention, service and 5-star treatment....
My point is...whilst an individual dive professional may have excellent merits, the location where they work is highly specialized and will operate in a very precise way. Training in one environment does not equip you to effectively step straight into operations in another environment....although it does allow you to transition over time, if your attitude is right.
However, learning in the environment you will be working in, is always the easiest and most logical strategy.