I have just submitted the application after more than 5 years residency in UK. I am required to send some documents together with the application including my passport. Do you know how long it takes to get the passport back from the Home Office? To note: I tried the Passpoort Return Service but it seems there are 4 weeks or more as waiting time and the documents must be sent within 10 days.
For all I know you will get your passport back when your application is accepted or rejected, which will easily take three months, and quite possibly longer. I would try to call around if there is a place where you can use the passport return service earlier, even if it is in a less convenient location.