Is it too late to wait for visitor in transit visa?

Hello everyone,
I have the following situation:
I initially bought the tickets for flight to Spain through the UK ( when going and coming back from Spain), later I found out that I need a Visitor in Transit Visa.
On the 10th of August I completed the online form and made and appointment(the earliest available appointment date was the 24th of August). On the 24th of August I went to the Visa Application Center where they collect the biometric data and some documents(like my passport, document from work that states that I'll have vacation from 14th to 27th of September, also the screenshots of the flight ticket).

On the 29th of August I received an email that stated that my application was received by ukvi.

Now my question is, will I receive my visa till 15th of September?

Initially I thought that the decision will be made in 15 simple days, today I've read that these are not simple days, but working days.
Now I have some concerns that I won't get my visa till my flight(if counting just working days starting with the 29th).

Thanks,
Denis.