Do I actually NEED a buy-to-let in this situation?

I currently live in a house I own outright (without any mortgage) which is valued at approx. £110K, and I want to raise a 70% mortgage on it to release some capital for a new purchase... And then move out and let it.

I have been researching buy-to-let mortgages and discovering they are much more expensive than standard residential mortgages... And then a thought crossed my mind... Do I actually need a BTL in this situation? Could I in fact just get a standard mortgage and ask the lenders for consent to let?

Does anyone know?
Thanks in advance :)