The Home Office strongly recommends against booking travel to the UK before a visa is issued.
Tier 5 visa holders must travel to the UK as soon as possible after the start date of their Tier 5 vignette. Those issued a vignette valid for 90 days must travel to the UK within this period, as if they do not, they will have to apply, and pay, for a replacement vignette before entering the UK.
Tier 5 vignettes are normally issued with a 'valid from' date around 14 days before the start date of the research visit, as listed in the Tier 5 Certificate of Sponsorship. This initial 14 day period allows the visa holder to settle in and take care of practical issues before they start their research.
Travelling to the UK as a Visitor
Applicants should also note that, once they have submitted their Tier 5 visa application, or after their Tier 5 visa has been granted, they must not enter the UK as a Visitor (e.g. as a tourist). If they do, their Tier 5 visa will no longer be valid, and they will not be able to start their research. Any travel plans should be discussed with the host department/ faculty/ college and the Staff Immigration Team (SIT).