Make sure you know what they are offering
Don’t be afraid to negotiate your starting point on the pay spine, but do be realistic as schools don't have much space to move in terms of pay.
A good head will make it clear what is being offered financially at the point they offer the job. So they might say, “We’d like to offer you the job of physics teacher, at £X,000 which is M2 plus whatever available weightings.” You are then very clear what’s happening. Some less-scrupulous heads will leave this out and you then find yourself starting on less pay than you thought. Look on the teaching union web sites for the pay scales.
Academies have their own contracts and pay scales, but be careful of the small print: one Academy Foundation in southern England makes everyone an employee of the Foundation and can send you to any of their 11 schools at any point!