As I am a Royal Engineer who is transferring to the canadian engineers, (albeit, I'm a canadian citizen already, so it's easier), I get asked alot what the process is like.
They will give you credit for all your qualifications/rank gained so far, but bear in mind there MAY be a requirement for attendance on certain phases of their training, simply to learn the kit/procedures etc. For example, I have to go to Gagetown for 2-3 weeks to make up for something I wasn't trained for in the Royal Engineers.
As for nationality, I won't lie, at the moment you need to be a canadian citizen, which as previously stated takes 3 years. However, and please take this with a large grain of salt, I heard from the careers office which was processing my application that they were thinking of removing the need to be a citizen, in the case of commonwealth trained personnel, ie ex HM Forces, aussies, kiwi's etc. But I have sent an e-mail to said careers office asking if this had in fact changed, and am awaiting a reply.
Bear in mind as well that joining the signals branch in the Canadian Army, in certain trades, may require a higher level of security clearance than what I need as a combat engineer. Which could lengthen the process of your transfer considerably. For example, for me to join a trade which doesn't, as a rule, require extensive security clearance, it only took a quick background check (2-3 months) and a UK Criminal Records Check. However, more sensitive trades, alot of which are in the signals, require 'Enhanced Reliability Checks' which take on average 18 months, sometimes less, most of the time longer. These must be done before you are offered a contract with the army.
I realise it's not the best news but if you have any questions feel free to PM me.
I was looking at transfering a few years ago but plans changed due to 9/11 and all emigrations into Canada stopping for a period however ..
I was currently teaching at Blandford at the time of applying and received a letter telling me they would accept me into the trade I was teaching however ...
I would have had to get out of the British army and move across under my own steam. Once in Canada I could apply again (showing letter of agreement) where upon I would have had to pass a BFT (their equivelant) and I would be taken on with my old rank (after a spot of 'conversion' training).
This method was a bit risky as I could have arrived to be told no thank you !!
There are quite a few YAHOO forums on emigrating to canada and even some for specific locations in Canada - good luck
Might I suggest that, if you want concrete answers, you contact the Defence Attache at the Canadian Embassy in London. He is not guarunteed to know answers to the questions you have, but he most certainly will know who to put you in touch with to find if it is possible, and if so, how to go about it.
Ref the security clearances, these might (and I emphasise might) be transferable if you hold UK clearances. Again, ask the Defence Attache, he should know who you need to contact to get answers on that, if it is an issue.