Hi waxweasel! I'm in Beckenham too although only for a couple of years, so I'm not an expert yet.
My son's due at the end of September. The thing I found really confusing here was the way the care is divided, so early appointments (booking etc) are at the Beckenham Beacon Antenatal Clinic, Later midwife appointments 28 weeks onward are with the GP, scans are at Kings in Denmark Hill and the birth takes place at the PRUH in Farnborough (Orpington area). This gets explained as you go along, rather than right at the start, which makes it more confusing still but after working out where I needed to be for each appointment my experience has been very good at all of them.
Having spent a week in the labour ward at the PRUH last year due to complications around an unsuccessful pregnancy, I have to say I felt very well looked after overall and the staff there were all very considerate and caring.
Do you have a GP yet? If not, I can recommend either of the GP's surgeries at the hospital.
I've just started attending the NCT antenatal classes in Beckenham and they seem good so far. I have and NHS one at the Antenatal Clinic in the Beckenham Beacon this Saturday too. The PRUH also does an early pregnancy evening where you can meet the midwives and also lots of local service providers, e.g. Bromley council's car seat safety team, prenatal yoga and hypnobirthing teachers etc. I think it's at 7 on the first Tuesday in each month.
You probably have most things you need from your older son but if you're looking for car seats, pram/pushchair etc. Baby Baby in West Wickham is a great independent local store with very helpful and friendly staff. We went there for our car seat and base and they gave a discount for payment in full over a certain amount too which meant it was also cheaper than buying from one of the big chains. Their clothes look lovely too; I haven't dared look at them too much as I think I'd end up buying too much!