I am looking for an excel download:
All active companies with a t/o of over £5m
Want trading postcode, turnover for the last year, and SIC code
Turnover is reported only by a small fraction of the companies that submit their accounts to CH. You can download the bulk accounts data files from Companies House data products - GOV.UK and parse the data at your end to extract this info. Not a trivial task though and pretty much requires a whole data transformation and aggregation pipeline to be setup, along with extensive data quality checks as there are noticable number of errors in the submitted info that needs correction during the processing.
Getting SIC code and post code is far more straightforward though, and the large CSV files available from the link contain this info - though it will likely be too big to view in Excel and will require any of the large file viewers(many free, opensource options are available).
Hope this helps