ragged my quote was about maths because it was sent from the Edexcel maths coordinator to maths teachers.
I've just found the DfE guidance and it says:
"The only English and maths qualifications that will count in the 2017 secondary school performance tables will be reformed GCSEs in those subjects or qualifications reformed to meet the same standards and expectations, the Department for Education said today.
This will ensure that there is consistency in terms of exams in these subjects, including linearity (exams at the end of the 2-year course) and limited non-exam assessment.
English language, English literature and maths are the first GCSEs to be reformed. They will be taught for the first time from September 2015, with pupils sitting exams in summer 2017.
The current arrangements for recognising other academic qualifications, such as level 1/level 2 certificates (sometimes known as IGCSEs) will end with the introduction of reformed GCSEs. Level 1/level 2 certificates in English and maths will not be included in the 2017 performance tables (due to be published in January 2018).
Similarly, level 1/level 2 certificates in subjects being reformed for first teaching from September 2016 (including history, geography, languages and the sciences) will not be included in the 2018 performance tables. This follows advice from the regulator about the challenges of including academic qualifications that could potentially be quite different from GCSEs in performance tables for 2017.
... Alongside these changes, the Department for Education confirmed that entries to the current GCSEs in English and maths from 2016 or earlier will not count in performance tables in 2017.
Schools may still enter pupils early for these ‘legacy’ qualifications, but if they do pupils will need to either take the new GCSE in 2017 or progress to a higher level qualification, such as an AS qualification, for their achievements to count in tables.
The exclusion of ‘legacy’ GCSEs from performance tables will apply only to English and maths, reflecting the weight placed on these qualifications in the new Progress 8 measure. As other GCSEs are reformed, we will continue to count achievements in ‘legacy’ GCSEs in all other subjects. This will allow schools to continue curriculum arrangements that allow students to take exams in some subjects - for example, 1 of the 3 separate sciences - before the end of year 11, having been properly prepared to do so."
www.gov.uk/government/news/qualifications-counting-in-future-performance-tables
So not counting early entry will only affect English and Maths, however for other subjects, you can sit GCSE early, but if you sit in 2018, you have to sit the new GCSE. IGCSE won't count.