The strongest case for switching off Slack in 2026 isn't that there's a dramatically better alternative — it's that if you're already paying for Microsoft 365, you're paying twice for capability you only need once. Slack Pro at £6.50/user/month for a 10-person team is £780/year of additional spend on top of an M365 subscription that already includes Teams. Five years ago, "Teams isn't Slack" was a reasonable defence of that double spend. In 2026 it isn't.
That said, switching costs are real. If you're already on Slack and the team is happy, the migration time and lost institutional knowledge can outweigh the saving. The right question for UK SMEs evaluating Slack alternatives in 2026 is "where would we be if we were starting today" — and the answer depends on which other tools you already pay for.
How to pick
On Microsoft 365 already: Teams (included).
Want free + casual: Discord (genuinely capable for small teams).
Want privacy / EU-resident / self-hosted: Mattermost.
Want best threaded-conversation UX: Zulip.
Already on Slack and growing: stay on Slack — switching cost is real.
For most SMEs already paying for Slack on the Pro tier (£6.50/user/month): if you're under 10 people and on M365, switching to Teams saves ~£780/year for a 10-person team. If you're not on M365, Slack remains acceptable.
The four worth knowing
Microsoft Teams. Bundled with M365 Business Standard (£9.40/user/month) and above. For SMEs on M365, the cost of Slack is purely additional. Teams is now sufficiently capable that the migration cost is low for typical small-team use.
Discord. Free for unlimited users. Originally a gamer-targeted platform, now widely used by tech communities, indie startups, and small remote teams. Voice, video, and screen-share included free. Strong community features, useful for community-led teams. Less business-tool integration than Slack; no HIPAA / regulated industry compliance; the aesthetic skews casual, which some clients may find unprofessional.
Mattermost. Open-source, self-hostable Slack alternative. Used by UK government departments, regulated industries, and privacy-focused organisations. Self-hostable for full data control and UK / EU residency. No per-user fees if you self-host. Operational overhead of self-hosting; less polished UX than Slack; smaller integration ecosystem.
Zulip. Open-source, threaded-conversation focused. Less mainstream but loved by engineering teams who find Slack's flat conversation model frustrating for technical discussions. The right answer for engineering-heavy teams that have outgrown Slack's threading.
How I'd actually pick
SMEs on M365: Teams. The bundled pricing is the right move.
SMEs on Google Workspace: stay on Slack if already there; Discord if just starting.
SMEs with regulatory or privacy concerns: Mattermost self-hosted.
Engineering-heavy teams: Zulip is worth a serious look — the threading model genuinely matches how engineers want to communicate.
What I'd swerve: paying full Slack Pro pricing if you're on M365. Teams costs £0 marginal and is now genuinely capable.
A note on switching costs
Migrating a chat platform isn't free even when the new tool is free. You lose searchable history (most platforms support some import; none are perfect). You lose integrations until they're rebuilt. The team needs a few weeks to settle into different patterns. For a 10-person team already on Slack and not on M365, the £390-£780/year saving from switching to a free alternative may be eaten entirely by the migration overhead in year one. Worth doing before you're 50 people; less obviously worth doing after.
Affiliate disclosure: Morningfold has affiliate partnerships with Microsoft, Slack, Mattermost. Verdicts based on testing — see editorial standards.