Read the separate boundaries
Ask about domestic allowance, speed policy, fair use and overseas data. A plan may have unlimited UK data while maintaining a travel allowance or conditions for unusually heavy use.
App-specific data benefits can have narrower boundaries. External pages, advertising, updates and unsupported activities may use general allowance. Subscription access to the app is another separate matter.
Evaluate the intended workload
A connection for video meetings needs stable uploads. Gaming may depend on latency. Backups can create sustained usage. A short speed test does not capture all of these needs.
For tethering and routers, confirm that the selected product permits the equipment and activity. A phone plan should not automatically be described as a fixed broadband replacement.
Choose from real evidence
Review several months of consumption, including planned changes in work or travel. Align device counters with billing periods where possible. Large differences deserve investigation.
Avoid choosing a larger allowance to compensate for a completely different issue such as poor indoor reception. Quantity, coverage and functionality need their own answers.