The honest answer to this search is that the main free route is HMRC's own, it is genuinely free, and it is limited by eligibility rather than by features. Everything else described as free tends to be free for one company, one year, or up to a size limit. Knowing which boundary you are near is the whole decision.
HMRC's own service
HMRC provides a combined service that prepares and files both the statutory accounts and the company tax return, with the iXBRL tagging handled, at no cost. It is aimed at companies with straightforward affairs: broadly a small company with a single trade and nothing unusual in its accounts. For a large share of owner-managed companies that is an accurate description, and those companies genuinely never need to buy anything.
Where the boundary sits
The service does not cover every case, and the published exclusions are the ones to check first: companies that are members of a group, companies with a chargeable gain to report, companies claiming R&D relief, companies with complex accounts or an audit requirement. A company that starts the process and finds it cannot proceed has usually crossed one of those lines rather than made a mistake.
What free costs elsewhere
Commercial products advertised as free are usually free at a tier: one company, or one submission, or a turnover ceiling, with the price appearing when the company grows or when a supplementary page is needed. That is not a criticism, it is a pricing model, and the thing to check before starting is what happens at the boundary, because the point of discovery is generally the week the return is due.
Questions people ask about free ct600 filing software
Is there genuinely free CT600 filing software?
HMRC's own combined accounts and tax return service is free and files both, with the iXBRL tagging handled. It is limited by eligibility rather than by cost.
Who cannot use HMRC's free service?
Broadly, companies in a group, companies with a chargeable gain, companies claiming R&D relief, and companies with complex accounts or an audit requirement.
What about free commercial software?
Usually free at a tier, with limits on companies, submissions or turnover. Check what happens when the company crosses that limit before you start.