Anybody thinking of moving to the US should have a long hard look at the costs of medical insurance, what it covers, and the horrific expense of any kind of medical care in that country. People try to come to Canada because, with all its faults, the Canadian health care system is so far not privatized.
People looking to move to Canada should also get accurate information on the costs of dental care - a lot isn't covered by insurance and the costs are very high.
An important factor also is the climate and the effect it may have on the quality of your life and the cost of living here. Climate varies greatly across Canada. Do you want to freeze (Alberta,Saskatchewan, Manitoba) get flooded (Manitoba), buried in snow (Maritime provinces) or burn up in a forest fire (British Columbia)?
Distances and the lack of public transport can also come as a big surprise. Having a vehicle is a necessity in most places, especially the colder parts of the country.
It's also important to get timely and accurate information on eligibility for Old Age Security, Canada Pension Plan contributions and payouts, RRSPs, Tax Free Savings Accounts and any other ways you can use to supplement whatever pension you may get from the UK. If you get a financial advisor, he/she should be qualified in Canada and able to advise you on how best to take care of your finances in this country. Hint: If he doesn't know what Corporate Class Funds are, he isn't on the ball - dump him.

Seems clear enough. But every year I get two P60s and both have tax codes that combined match the personal allowance, but they are split so as to cover both gross amounts.

The very first payment was done slightly differently and tax was deducted but then refunded when they began using this method.

Seems a funny way to go about it if there was an identifying code to enable gross payment without needing two different codes.