Best way to invest £100k – quick poll

As an unscientific personal vote I go

Ltd Co Purchase

But then I did add the picture 🙂

No brainer - option 4

Assuming you mean poll not pole, I would say depends entirely on your own personal circumstances, e.g. age, other liabilities, amount outstanding on own residential mortgage etc. and attitude to risk, but most likely Option 4 which will give you security and peace of mind with improved cashflow.

Not knowing what's around the corner (and what other crazy ideas this government may come up with), personal security can hardly be a bad thing...no matter what, you will always need somewhere to live.

Option3. Paying down BTL loans will provide the better financial return - interest rates for BTL are higher than for your own property, and the return is even more if you're hit by S24. Reducing your borrowings may also help you obtain a lower interest rate at renewal. Unless you live close to an area suitable for holiday lets, then go for option 1 when you feel the general upside outweighs the risks.

option 1. Theoretically I have enough income during retirement but want manage leaving things to kids.

Personally if it was me I would go for Option 5 ( and not just because nobody else has said it ). I currently have a residential mortgage ( where I will never move from ) and 6 buy to lets. If I had the extra £100k I could in my area purchase an ex local authority house out right and rent for approx. £550 per month. I feel this would compliment my existing portfolio. The other issue with me is that I have no other income as this is my job so the section 24 is not going to cause too much ill effect.
It does depend on your personal circumstance.

Just to be different, you could consider option 2, the holiday home. We started letting ours on 1st July last year and already had 130 days of bookings, so already well over the HMRC 105 day requirement in our very first year. Consider all the advantages. Cost of set up for furniture, carpets, bedding, towels etc are all fully tax deductible as capital allowances.
No council tax to pay - you pay business rates and for most owners this is set at zero.
Forget Section 24, if you take out a loan it will be fully tax deductible.
When you sell, a lower rate of 10% capital gains tax applies.
The income counts as earnings so you can pay pension premiums and get tax relief.
You will usually net, after all expenses at least double your income from a similar property let on an AST.
You can use it yourself for personal use. (we just use ours in likely void periods)
You can get companies which will fully service your house/flat on changeover days. Somethings which seem a major problem aren't really. You can get affordable insurance which will will cover most eventualities including your place being trashed. (unlikely if you choose smaller properties for family lets rather than big properties attracting hen and stag parties). My holiday insurance even includes theft by non-forcible entry. You obviously need a good location, not necessarily on the coast, many rural or semi/rural areas are very popular.

Have you considered a variation of your number 4, paying off your own mortgage.
See if your current lender offers an offset mortgage facility. That way you will be saving the interest due on the first £100k of your mortgage and still have access to the capital should an investment opportunity arise.