Your first 30 - 35 hours will be in sim, this will not give you any delays. However you have to remember when you go into aircraft you have issues like : FTO normally have few students to schedule, aircrafts goes tech, airfield equipment tech (ILS/NDB calibration) happen to me on my test day, so best laid plans got changed on test morning, weather as mentioned, booking of 170A - time and schedule, booking of examiner with the CAA.
From I did by 170A, it took 3 -4 weeks before I was able to get my test day booked in with the CAA, only to cancel on test day due to weather, and because of aircraft availability and examiner, it took another 2 weeks to do the test.
So 2 months is pushing it very tight, normally FTO does not do more than one sim session a day, becomes counter-productive after this, you need to process what you need to do. If you do IR on a DA42 - G1000, there is some excellent add ons for Flight Sim, that can get you practice your procedures at home.
However if you want to have a buffer zone for unexpected events, I would say 3 - 4 months would be ok, anything less, and it might just be beyond your control. Those 15 hours in the aircraft seems a little and fast to do, but depending on luck/unluck who knows. Also this summer if you in south east region, there are some restrictions due to the Olympics, not sure what they are, but heard some news of this. 2 months is not safe period for your IR, I am sorry.
What you could do, find an FTO where they do the sim sessions, and you could start that early part time, the sim is not so expensive, and when you have completed that you can start in the aircraft during your 2 month period.
I think the normal average period is 3 months from start to finish. But do not forget from your 170A pass to your test, this is beyond your control, and you do not want to end in a situation where you can not cancel your test because of to much wind.
If on test day you have wind above 20 kts, or and wind gusts which is not unusual in the UK, you will want to be able to cancel your test and re-book it, if you running out of time you might have no choice, and it will show bad airman-ship if you decide to fly under to bad conditions. Will be a pity if you have already spent all the money on the IR, and you run out of time because of this.