Remortgaging 1 bit of multiple mortgages

Krispycing

Cant see much help on this subject - perhaps someone can guide. 
My situ:

Home worth approx £450k
Purchased in October using the following: 

- Equity of £90k
- Mortgage A. (ported from prev. house) £280k - 3.5 yrs remain on 5 year fix
- Mortgage B. (taken at time of buying new home) - £80k - 1.5 yrs remain on 2 year fix

I am looking ahead to when I have to remortgage. My plan was always to remortgage Mortgage B when it's fixed term ends.
The idea being to get a new 2 year fix at that point (lets call that one Mortgage C )

- which would then mean that when Mortgage A fixed term ends, Mortgage C fix term will be pretty much finished too and I can, at that point, combine them both into a single mortgage - which I will call (yes, you guessed it, you clever devil) Mortgage D.

Notes:

- I might have to float about on SVR for a month or 2 depending on timing but I can handle that (hopefully!) 
- I will have paid some of the above off, I am using values as they are now rather than as they will be - think the principles are the main thing here rather than getting into specifics.
- I plan to overpay but I dont know how much by. 

Q. How/do they calculate the rate for a mortgage in this scenario? Require C = 80k for 2 years (but then C would have LTV = 18%?!)
That cant be right, so it must include amount from A too - ie. £360k forms basis of LTV = 80%) - I don't see this catered for in calculators. I hope its possible. I was suddenly aghast today at the prospect of spending 2 years on SVR after B ends. Can someone set mind at ease? Thanks !

Leah04x

The whole amount owing over all loan parts is taken into consideration. So if you rate switch mortgage B on to a new 2 year fix, they’ll offer you a new product based on the full £360k mortgage against the £450k value. 

penners324

You'll only be able to rate switch with same lender. The lender will offer a number of options to switch to.

Krispycing

Thank you both 🙌🏻