Overview
A sponsored worker's right to enter the UK, start work, and travel after being granted a visa is governed by the dates and format of the visa itself, not by the CoS dates. Sponsors should align start dates with the visa valid-from date and should understand the difference between a vignette and an eVisa.
The visa valid-from date governs entry and start of work
A Skilled Worker visa has a start date (valid from) and an end date (valid until). The worker cannot enter the UK or begin sponsored work before the valid-from date, even if the CoS start date is earlier and even if they have arrived in the UK on a different visa category. Sponsors should not allow the worker to start before the valid-from date, as this is unlawful working and a compliance breach.
Vignette versus eVisa
Visas granted from overseas typically come with a vignette (a sticker in the passport) valid for 90 days to allow entry to the UK. On arrival, the worker registers for an eVisa, which then evidences their leave for the full duration through their UKVI online account. Visas granted in-country are issued as eVisas directly, with no vignette.
Biometrics for dependants arriving separately from the main applicant
Where a main applicant has arrived in the UK and the dependants are following later, the dependants will typically need to attend biometrics at a visa application centre abroad before travelling. Dependants cannot complete biometrics in the UK unless they are already here on another visa and switching in-country.
Arrival ahead of the start date
If a worker arrives in the UK ahead of the visa valid-from date on another visa, such as a visitor visa, they cannot begin sponsored work before the valid-from date. They may also be required to leave and re-enter the UK if their current visa does not permit an in-country switch to Skilled Worker.
Travelling during an active sponsorship
A sponsored worker can travel in and out of the UK freely during their visa validity period. Short periods abroad do not end sponsorship, but sponsors should be aware that extended absences (such as remote working abroad for months at a time) may affect the worker's eligibility for ILR under the 180-day absence rule.
Travel during a pending application
A worker with a pending Skilled Worker application should generally not travel outside the UK while the application is being decided. Leaving the UK is typically treated as withdrawing the application.
Government guidance: https://www.gov.uk/skilled-worker-visa