Issue and Background
Having gotten a new job, I am moving out of my rented accommodation, and moving across the country to what is a far more expensive area, where I am buying a 45% share in a £260,000 shared ownership apartment (Rent-only properties in the area in question are quite frankly shocking in price).
Due to a technicality which I am fine with, I am moving from a mid-level role at my current workplace to a junior role at my new workplace. Even with moving to a junior role, my salary will increase from what I am earning in my old job (due to complications at the old job). In addition, I have been told it is likely that I will be promoted back to mid-level within 6 months to 2 years once this technicality is overcome.
Unfortunately, the difference in monthly salary between my old job and my new job, is less by quite a bit than the difference between the rent at my old place and the monthly cost (rent + service charge + mortgage) of my new place, assuming I get a standard 25 year mortgage. I can manage (without becoming hand-to-mouth), but I would have to cut down on quite a bit of my monthly outgoings. My planned trip to Japan would probably go out the window too.
My mortgage advisor/broker has suggested an alternate plan of action which I was quite surprised at, as it is something I had never considered, and I cannot find mentions of anywhere on the internet.
Given I will be remortgaging once the fixed term of whatever mortgage I get now ends (to avoid going on the standard variable rate), go for a 40 year (the maximum possible lending term) mortgage for that first fixed term. Once that fixed term period (likely to be 2 years) ends, remortgage onto a much more standard 25 year (or less) mortgage.
This keeps my increase in my monthly outgoings for those first 2 years almost exactly in step with the increase in my monthly salary between the jobs (accounting for tax, etc). It also protects me in-case the worse happens and I lose my new job in the first two years, because the monthly cost is lower (I have no reason to expect this will happen, it is just extreme-worse-case planning). Then, by the time the remortgage is happening, I hopefully will have been promoted and have more money available a month (but nor am I reliant on that happening either).
* Both mortgages are with the same company and are identical except for the term.
As a first time buyer, Is the advice of taking out a very long term (40 year) mortgage, with the plan to remortgage to a much lower term mortgage once the fixed rate period of the first mortgage ends (and thus can have hopefully come into more money and as such can much more easily afford it), a sensible option? Also, are there any probably caveats?