OP, surely you can understand why you've got people's backs up? You're talking about starting a family (not "having a baby" btw), yet you and your partner are not even a family in your own right yet. You've talked about moving in together, but haven't actually done it. Thinking you're ready for a baby while still clinging to your mum's apron strings is a bit weird tbh.
Also, you referred earlier to your boyfriend being "petrified of losing you". Really? Why? Mine isn't. Why would he be? We're a team. Are you absolutely certain that he's not just going along with your plans because of this?
But onto the advice...
The first thing you absolutely need to do is find a place to live. As a family you will want, nay NEED, your own space. As a mum, you'll need to find your own way and while it will be fabulous to have family support nearby, you'll do this better without having someone constantly watching over you.
You also need to have a very clear and considered look at your finances. What will rent, council tax, utilities, insurance, food, petrol, baby necessities etc etc cost per month. And how does that stack up against your/your dp's salaries? Also what benefits will you be entitled to to top things up if necessary (working tax credits, child benefit, statutory maternity pay, housing allowance etc)?
It will be important to check your work contract to find out what maternity benefits you are entitled to. Do they offer an enhanced package? (You do have a contract, right? If not, you need to get one pdq - please don't rely on assurances from your employer!). You need to have worked a minimum of 26 weeks in the run up to your due date in order to qualify for SMP btw.
Assuming all these costs work out, also consider what you'd do if circumstances changed. There's a general election in 11 months' time. If the Tories or, God forbid, UKIP get in then benefits for unemployed or those on low incomes are likely to drop even further... how will you cope if that happens? You're lucky in that you can go back to mum and dad, but that really should be viewed as a last resort (as it would be for most adults) not as an automatic back up.
Finally - I'm not saying all this just because you're young. DH and I left highly paid jobs in London to move to the countryside. I started my own business, he took a less stressful job on lower pay. I won't be entitled to SMP, just maternity allowance, and we're not entitled to any benefits, so I've had to do all the calculations above to make sure we can afford to do what we're doing. If we couldn't then we'd have to make changes - and that wouldn't mean moving in with my parents. That's what it means to be an adult.