Why does there have to be a "next" mortgage company? Is your existing lender so bad?
Yes, you should, though you could get away with not doing so. Expect to pay a premium of about 1% on top of normal interest rates for the privilege of being "allowed" by your lender to do this. If you don't tell them, and they find out, they may require you to repay the loan immediately. It is unlikely, though.
More importantly, you need to tell your insurer, and you may even need to change insurer because not all insurers will cover non-owner-occupied houses. The new insurer may tell the lender...
Not necessarily. Since you expect to return in 3 years, you will be reverting to standard anyway, won't you, so you may as well stay on standard-plus-permission; permission is unlikely to cost more than the difference between typical BTL and typical owner-occupier loans. If you're thinking of changing lenders anyway, your best bet may be to consult a mortgage broker.
Plenty, no doubt, but I won't comment. As you're going to be abroad, then unless you have friends/relatives who would be prepared to keep an eye on the place for you and sort out tenancies, etc, or unless you already have someone lined up who will be living there for the duration, you really ought to consult a letting agent who will then manage everything for you. Often letting agents will be able to recommend a mortgage broker savvy in BTL lenders. They may even share the same office!
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