SMS gets 95%+ read rates and click rates several times higher than email. It also costs around 4p a message in the UK and lands on the most personal device your customer owns. Used well it is the best-performing flow channel in the stack; used badly it earns unsubscribes, ICO complaints and a reputation you cannot buy back. This guide is UK-specific because most SMS advice online is written for the US, where the rules are different.
What UK law actually requires
- PECR (Privacy and Electronic Communications Regulations) governs marketing texts. You need prior explicit consent for marketing SMS to individuals. The "soft opt-in" exception (existing customers, similar products, easy opt-out at collection) exists but is risky for SMS; treat explicit consent as the standard.
- Consent must be specific to SMS. An email opt-in does not cover texts. Your form needs a separate, unticked SMS consent with clear wording.
- Every message must identify you and include an opt-out. "Reply STOP to unsubscribe" or a link. Klaviyo appends this automatically on the first message and periodically after; include it on promotional sends regardless.
- GDPR applies to the data. Record when, how and to what wording each person consented. Klaviyo stores this per profile.
- Quiet hours are not a hard legal rule in the UK, but sending between 21:00 and 08:00 generates complaints. Set Klaviyo quiet hours to 08:00–20:00 UK time.
"Yes, text me exclusive offers and order updates. By ticking this box you agree to receive recurring automated marketing texts from [Brand] at the number provided. Consent is not a condition of purchase. Msg & data rates may apply. Reply STOP to opt out. See our Privacy Policy." Keep the checkbox unticked by default.
Setup in Klaviyo
- Sender ID. In the UK you can use an alphanumeric sender ID (e.g. "SleepyUK") — no replies possible, but higher recognition. Use a long code or short code if you want two-way conversations. Alphanumeric for most brands.
- Two-step sign-up form. Email first, SMS as an optional second step. Adding SMS to the same step reduces email sign-ups.
- Checkout consent. Shopify's checkout "Text me with news and offers" toggle syncs to Klaviyo. It must be unticked by default for UK stores.
- Compliance settings. Enable quiet hours, set the country to GB, and turn on the STOP keyword handling (default).
- Link tracking. Use Klaviyo's short links so you get click data and can attribute revenue.
Which flows deserve SMS
| Flow | SMS role | Timing |
|---|---|---|
| Abandoned checkout | Message 3 or 4 of the sequence, after two emails | ~24h after abandonment |
| Welcome | Deliver the sign-up incentive code instantly | Immediately on SMS consent |
| Back in stock | The best SMS use case — urgency is real | On restock |
| Shipping & delivery | Transactional; consent rules differ, still keep it useful | Dispatched / out for delivery |
| Replenishment | "Running low?" for consumables | Predicted reorder date − 5 days |
| Win-back | One message, real offer, after email has failed | Day 60–90 |
Do not put SMS in browse abandonment — it feels like surveillance. And keep SMS to one message per flow, not a sequence.
Campaigns: less is more
Two to four campaign texts a month is the ceiling for most brands. Reserve them for: early access to sales (see the BFCM playbook), launches, and genuinely limited offers. Segment to engaged SMS subscribers (clicked in 90 days) plus recent purchasers; send to the whole SMS list only for your biggest moments.
Copy rules
- Brand name first, one sentence, one link, under 160 characters where possible (multi-part messages cost more and read worse).
- Use the customer's first name only if you have it reliably — "Hi ," is worse than nothing.
- State the offer and the deadline plainly. No emoji walls.
- MMS (image) messages cost 3–4× more in the UK; use them only for product launches where the visual is the point.
Costs and what to expect
UK SMS through Klaviyo costs roughly £0.035–£0.05 per segment depending on plan and volume. A 5,000-subscriber list sending 3 campaigns a month plus flows spends around £600–£900. Benchmarks we see on UK Shopify accounts:
- Click rate: 8–15% on campaigns; 15–25% on abandoned checkout SMS.
- Revenue per message: £0.30–£1.20 on campaigns; £2–£5 on abandoned checkout and back-in-stock.
- Unsubscribe rate: under 1% per campaign is fine; over 2% means frequency or relevance is off.
- SMS list size: typically 15–30% of your email list after 6 months with a two-step form.
Track SMS revenue separately in Klaviyo's channel reporting and fold it into the email share metric from our KPIs guide. Combined email + SMS revenue share above 35% is where well-run consumables brands sit.
SMS earns its place by being rare. The moment it feels like email with fewer words, the channel is gone.
Frequently asked questions
Is SMS marketing legal in the UK?
Yes, with prior explicit consent under PECR. You need a separate, unticked SMS opt-in (email consent is not enough), clear identification of the sender, and an easy opt-out in every message.
How much does Klaviyo SMS cost in the UK?
Roughly £0.035–£0.05 per SMS segment depending on plan and volume; MMS is 3–4× more. A 5,000-subscriber list typically spends £600–£900 a month including flows.
Which Klaviyo flows should use SMS?
Abandoned checkout (as a later step), welcome incentive delivery, back-in-stock alerts, replenishment reminders and a single win-back message. Avoid SMS in browse abandonment.
Can I use an alphanumeric sender ID in the UK?
Yes. UK networks support alphanumeric sender IDs (e.g. your brand name). Customers cannot reply, so STOP handling works via link instead; use a long code if you want two-way SMS.
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