You should be entitled to SMP from your employer. It doesn't matter how big or small they are, as long as you are actually employed by them (not self-employed, some small employers won't do PAYE and basically make you be a sole trader and you are treated as self-employed for tax purposes, you should know if this is the case and then you would have to apply for maternity allowance). Otherwise, it should be SMP through your employer assuming you otherwise qualify (you will need to work up until your qualifying week though)
I've never done SMP, but have applied for maternity allowance twice now (the second by as an employee, but my contract ended before my qualifying week, so I wasn't eligible for SMP). The form is really simple and self-explanatory, but I would pursue the SMP route first unless you are self-employed.
Keep in mind though the earlier you start, the earlier it runs out, so that means you may need to go back to work sooner than you want unless you plan to be a SAHP after. If it was me, I'd get the SPD treated (osteopath has pretty much sorted me out in all my pregnancies and after two appts, I felt great again), have them accommodate any needs you have (to sit, sit on an exercise ball, stretch, rest, whatever), and carry on working to bank as much time at home as you get get with baby. I was able to carry on until 36 weeks with my first and plan to do it again this time (I'm 30 weeks now).