You're obliged to pay
UK tax if you are resident in the UK, the rule is that residency begins either (a) if you are in the UK for more than 90 days or (b) as soon as you start work and have income. The only way around it is to claim exemption via the tax treaty. Where you reside is more important than where you work as you are taxed on worldwide income.
It can get complicated because Canada has a really tough definition of "residency" for tax purposes, you basically have to cut all ties, such as DL, healthcard, address, etc. in Canada, plus there is a capital gains tax that can apply to people who leave permanently, check the CRA non-residents website.
Talk to HMRC, they have a website for new residents. Bear in mind the UK tax year is different from the calendar year, it runs from April 6th to April 5th.
My sister is an airline pilot and she tells me that HMRC has a special deal with the airlines, because you work in international airspace you only have to pay 10% of the income tax. Not sure how that applies to other aircrew.
Steve.