A Each case is decided by employers on its merits. Since those who train you will not be your employers, you will not generally know until after you qualify whether or not a school will recognise your experience with extra payments.
However, it is worth reminding schools that they have the right under the teachers' pay and conditions document to pay up to "one point a year for years of experience other than employment as a teacher" that they consider "of value to the performance of a classroom teacher's duties". The Department for Education and Employment circular on pay and conditions specifically includes non-teaching experience, whether paid or not, in this category.
Sometimes schools would like to pay more but don't have enough money in their budget to do so. I'm afraid it is still a bit of a gamble. As teacher shortages are often a their worst in parts of London that can be an extra bargaining point in your favour, but is no guarantee.
One other solution would be to see if a London school will take you on under the Employment Based Training Scheme so that you can be paid during the training period.
Q I graduated from university last summer (2:2 in history) and fancy a year or two of teaching before I start a career in the city. I certainly don't want to do a PGCE. Can I get a teaching job in the private sector?
A These days it is normal to expect to undertake some form of training before entering any profession and teaching is no exception. Although a PGCE is usually a one-year full-time course, there are some part-time and modular courses available.
Schools in the independent sector are not required to employ trained professionals but most do, for obvious reasons. They may make some exceptions if you have some other skills to offer.
You might want to contact one of the agencies that specialise in staff appointments to schools in the independent sector and ask their advice.