According to Amazon UK the 20GB PS3 may be in the UK sooner than we thought. It's just appeared on the website with a release date of "30 Oct 2007".
SPOnG contacted Sony to see if the UK will, in fact, be getting any lower end PS3s before the end of the year. While not giving a definite 'no,' Sony's head of UK PR, David Wilson, said of the 20GB console "I haven't seen any. We don't have any here."
The date shown on Amazon may well be a place holder. It would certainly be an odd choice by Sony to begin advertising the cheaper PS3 on the same day as the 60GB model goes on sale.