No one said it was easy but you said you'd like to have it done and dusted in 6m in order to save money on mortgage and other payments.
This is wishful thinking.
The lender has a vested interest in your property, till you pay it off its basically there's.
You apply to subdivide via land registry then the lender will re-visit and see if by losing part of the land from the oropertt affects the value of what's owed and what percentage of equity is left in the property.
If this is low they will refuse your request. You will then have to stump up some money so make sure prior to you taking out your mortgage you can make overpayments with no penalties.
The mortgage company want paying for this BEFORE they lend you any money on the property, regardless of whether you buy it or not.
This way you can overpay and drive down what's outstanding on your mortgage in turn making it possible to split part of the property you want to subdivide.
Yes granted there are lots that can go wrong along the way but this doesn't mean that every case will be a mare.
I'm not one for not doing because someone has a bad experience I try to learn from those mistakes and hopefully spot problems before it's too late.
You never know till you try
You seem to be asking for approval for a hare brained scheme and instead of listening to advice, you are seeking ways around any advice you are offered.
No one, apart from you, knows the sums involved, but you stand to lose tens, maybe hundreds of thousands of pounds.
It doesn't make a blind bit of difference to me or anyone else here whether this happens or not but it will matter to you.
If a £150K house comes up for sale near me and I think I can sell off the back garden for building of 4 bungalows so I get a mortgage for a fortnight and make 100K on a land sale, do you really think that every person in the UK wouldn't have caught onto the same idea?
When properties with large land parcels come up for sale they are snapped up by medium sized property developers who bulldoze the house and build four or five plots.
They don't buy on a mortgage.
If you have some money you don't need buy a house on a mortgage and rent it out.
Go back to the bank and show that you own several properties and they will give you as many mortgages as you want as they can't lose.
Neither can you as rent and property prices never stop going up but mortgage payments never stop going down