Can anyone answer my question? I couldn't find information on the web, and didn't want to ask my agent. Maybe, I'll get the answer here.
Is there any law stating one must go with a mortgage on the origin of what he/she plans to do with his/her property.

For example, is it necessary to go with a buy to let mortgage, if I plan to purchase a property understanding I will ultimately let it? Or can I go with a normal mortgage and let it in any case?

Do I still have the ability to get a rent guarantee cover without a buy to let mortgage?

Usually banks are o.k. with "change of circumstance" as long as a nice courteous letter is written in beforehand of the event taking place.

For instance, buy with the purpose of living there - certain months after conclusion admittedly you have to move out to take care of an ill relative. Before you officailly move out, ask for permission to move out on an ephemeral basis. Of course you will not have the intention to sell as you could move back soon. Ask them for provisional permission to let.

That is so right! This way you have got the best of both sides. On one hand - residential rates. An on the other - conditions (including higher LTV's), no rental evaluation initially. And it is all legal as long as your 'aim' is to move back in.
Really, a good suggestion

It is dangerous to do such thing.
Before letting your property, you have to receive your mortgage lender's permission.
You will be breaking your mortgage conditions, if you default to receive consent. In this manner you could get a 'black mark' if the lender ever finds out.
So, it's better to think once again before doing something like this.

Why is it dangerous? If you have the lender's permission to rent out your house on residential mortgage, why should you waste more funds on buy to let mortgages. Of course you will not have some options and landlord insurance. But, if you simply want to save money, that is a great option.

If you already have this property for sometime, lenders are likely to give you a permission to let, but if are just buying it and already have another residential property be careful. If they refuse to give permission to rent it, you will be in a breach of contract and they have a right to recall the loan.

At last, I feel that I could positively contribute to this forum! My circumstances changed recently and I will have to relocate to another country for a period of time. However, as I intend to come back, selling my property was not an option. My lender was very helpful and I do not expect to have any problems in securing their permission to let my property out.

If you are applying for residential mortgage known full well that you will not be residing in that property, then you will be misleading your prospective lender. Of course, if your circumstances do change, you can approach your lender for permission to let. In my experience, it does not often present a problem.