Remortgage just months after mortgage

proofoffunds

I will try to be short:
Current situation: I have an apartment costs 220k I bought it just 2 years ago, and now I still need to pay 180k. I can pay it in full without any additional fees when I want.
I plan to buy a house that costs 400K. I already spoke with financial advisor and he think there will be no problem with a new mortgage. The plan is to sell the apartment just after that or around the same time (I don't like the idea of the chain so I plan to avoid it if possible even if it's more). Now become the question itself - In the beginning I will have only 10% deposit, so I will use a mortgage ~3.5 interest for the first 5 years. But when I sell the old apartment I should have something like 40K on top of everything that I already paid to the new mortgage. In this situation I can very fast have less than 320K to cover from the new mortgage - less than 80% of the value of the property. 
When this happen is it possible/reasonable to remortgage immediately to drop the interest rate to 1.9%. I know I will pay some remortgage fees, but I will pick a mortgage product that allows overpayment without additional fees just to be able to do that when I want.

Thankyou

julicorn

From what I've seen, remortgage products generally need you to have owned the property for at least 6 months, but I'm sure the brokers on here will have a better idea. 

Thrugelmir

If I understand your plans correctly. Do you meet affordability criteria for holding both mortgages concurrently? 

proofoffunds

julicorn, 6 months are not a problem. I can wait even 1 year if this is a requirement.
Thrugelmir, generally yes. But now I find out that is pretty hard to take 2 mortgages in the same time and the rates and minimum deposit are different.