You should probably chat with a lawyer about the options here. However:
Intention of the Condition
My understanding is that the "work requirement" is there to ensure that you are willing and able to support yourself without recourse to public funds. It is also to stop people using it to retire to the UK, or living in the UK as persons of independent means. The latter cases should be using different visa options.
Employment Options for Ancestry Visa Holders
Based on historical discussions with my lawyer, the purpose of the Ancestry visa is definitely not tied to working for a UK registered company, unlike some of the other visas.
On the ancestry visa people can be self-employed and start/run their own companies, or even work part time and odd jobs (assuming they can make a reasonable living doing so). These would all be valid routes of "working in the UK", and could be considered valid. The main requirement is that you have some source of work/income that the immigration officer can feel confident in.
This is one of the points where the immigration officer has some leeway to decide, so it's up to you to convince them with a reasonable argument as to what you plan to do and how you are going to make a living. This could include your previous employer offering to carry on having you work for them, in my opinion.
Also note that immigration officers consider "positive feedback on job applications" and "salary survey results" as useful evidence when applying for the ancestry visa. My wife and I came in on the ancestry visa without a specific job offer, but lots of job-website submissions and lots of positive emails/feedback from potential employers. My understanding is that you could provide this AND letters from your previous employer (on a company letterhead) saying they will keep paying you to work remotely. This would help convince the Immigration agent, in my opinion.
The UK Border Agency publishes their internal guidance notes on the Ancestry visa. They aren't particularly verbose on this point, unfortunately, but the other sections may help you.
Tax and Financial Implications
One further point here that I think you should consider is the tax and forex implications of working for a foreign company. You are likely going to be subject to $/£ currency fluctuations, since the company will probably want to pay you in $.
You may also find that you are still taxed in Canada, and need to jump through complicated hoops so as to not pay excessive double-tax. The previous employer in Canada may also have limited ability to not tax you as if you were resident in Canada.
If you are business savvy (and can prove that to the immigration agent), you might want to consider setting up your own company, or possibly using something like an Umbrella Company Provider. I'm not sure if the Umbrella companies allow for overseas work, though. You can potentially use this to simplify things for your previous employer, and to clarify your tax situation.
The company above, 1st Contact deals with foreign nationals arriving in the UK, setting up companies, giving immigration advice, and similar. I was their customer for about 3 years, and then moved to a different division of the same company. (Other than being a customer, I'm not affiliated). You might find they can advise you on this sort of thing.