This went back up to £24.99 after I last posted it but noticed it was £19.99 again this week (in store, Redditch). I guess it will be that price nationally where stock remains.
The rush of clearance reductions on the older (Ni-Mh battery) Oral B toothbrushes seems to have subsided now but rarely got cheaper than this anyway. If you missed out you might want to try Savers: the Redditch store had a pile of them reduced to £19.99 (previously reduced to £24.99). The 650 is the kit with the 600 brush plus a tube of Oral B toothpaste. They were available in black or pink. Still £24.99 in B&M and elsewhere last time I looked (and £50 in Tesco - ho, ho, ho!).
The only real difference between these and the much more expensive Li-ion ones is that a charge will last a week vs two weeks for the newer ones*.
The 600 is the simplest model your dentist and and hygienist would recommend - it has 3D action, which is important. The Vitality model is 2D only so it's not nearly as good.
*Top tip: buy two and keep one on charge while the other is in use. That way you never have to wait for one to charge (whichever battery it has) and it's still cheaper than a Li-ion one. You'll get two brush heads and two tubes of toothpaste too!
I just had to have a tooth implant and I'm still in shock - £2600!!!! I'm following the dentist's recommendations very closely now and it makes these brushes seem cheap!