Aw Samira, it's a tough one!
You seem to be leaning towards giving them notice to leave now, and I totally understand why, however, here's another way of looking at it
The baby can sleep in a moses basket for up to 6 months - I know you said there isn't room for a cot in your room, but is there room for a moses basket?
If your tenants move out, lets say, end of September, your baby will still only be 8 weeks old & they don't really need much "stuff" at that age.
You'll be on maternity leave (I assume) & can go & sort out the nursery in your house, which will be freshly done when your baby is ready to move in to it
Plus, you'll have made an extra £5700 in rent, between now & September!!
Xx