Reality checks very much needed. Devils advocate in play here and some of these points may touch a nerve but have to be considered.
We’re currently living apart, she’s in Nottingham and I’m up in Sheffield. If it comes to the worst and we’re going to struggle to get a mortgage then I would ask my parents if they could out us up for six months while we save that bit more which would enable us to get a mortgage. Not ideal but better than renting and saving not much at all p/m.
Living with your parents with a new born baby no matter how awesome they are will put so much extra pressure on you all. I would advise against it unless your parents have a granny annex and you can have your own space, comfortably.
Lets not forget as well that your girlfriend will be living in a house that isn't part of her side of the family and she will be there most of the time while on maternity. It never feels like your own space.
She does have a permanent contract and I'm pretty sure she get's a carers allowance but I would have to double check that. Living with my parents is a back-up plan, although I've not asked yet (no-one knows about the baby until we make it official) I'm fairly certain my mum and dad would happily put us up on the basis we pay our way.
So the carers allowance would stop if she was no longer caring for her mum... can you really expect her to be doing a 50 mile each way trip with a new born baby each day or multiple times per week to continue this commitment, and if she did, the petrol alone would be crazy.
You also both sound young, how happy are your parents really going to be with the fact you have never lived together and you've fallen pregnant?? What happens if they basically wash their hands of you with you for being so irresponsible?
Living together at your parents will be a great relationship stress test, if you can both stay together through that then you should be fine in your own place

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On the positive side you'll have live in babysitters / extra hands).
If only it worked out that way! On the flip side you will have a live in "self proclaimed expert" getting involved in the raising of your new born baby and you are in the difficult position of not being able to tell them to P-off as you are living in their house so need to keep the peace.
You also have to deal with the guilt factor of imparting the thrills and spills of a new born baby on your family. Sleepless nights are bad enough for new born parents but when you have the added guilt of causing sleepless nights for others in the house.
That's what is putting me off renting.
As others have said, renting is a lot easier to get out of if things don't work out and whilst you wont be able to save as quickly for your first house, you will be able to rent and save still.
The fact you haven't lived together, live 50 miles apart currently, she's pregnant (I hate to ask but are you even 100% sure its yours?), and both of you live at home with your parents currently all sound like very good reasons not to buy a house now.
What happens in 6 months time after moving in together you actually realise it isn't working?
Not to mention its a lot cheaper to get in to a rented property, and have your own space with much less hassle.
I am a staunch supporter of buying over renting but you have to many unknowns to factor in.
Save until you move in to rented, then continue to save at least £100 per month over the next 2 years in to the HtB isa and you will have an extra £3k saved.