Why would you need to wait a month to exercise? Start now.
I find it improbable that you cannot find at least £15 a month to pay for a gym. Anyhow, if you're not gonna go to a gym, at the very least buy a pull-up bar so it gives you more options in terms of compound body-weight exercises.
Also, push-ups, ab crunches and body-weight squats will not build you muscle (efficiently). For one, they are mostly endurance exercises and it is hard to progressively overload with them. Google some calisthenics body-weight exercises that involve compound movements involving multiple muscle groups, dips, pistol squats, pull-ups, chin-ups, etc.
I will say this, if you want to put on mass quickly and efficiently, go to the gym and use a barbell.