In the UKVI Operational Forum that took place on 18 May 2015, these minutes were recorded. Please note that they are paraphrased and not direct cites...
Attendee -What happens if you don’t travel within 30 days?
PS -If you don’t travel within the 30 day validity of the vignette you will need to apply for Transfer of Conditions (TOC) vignette, costing around £140ish (note: subsequently confirmed that current fee is £122). Where an applicant is aware that they will/are delayed in travelling it is important that they alert the local post as soon as possible. We will liaise with international colleagues to identify applications and ensure that the card stays in Post Office. If the card is recalled from the Post Office as it has not been collected and it is destroyed then a fee to replace the card will be required of the applicant.
Attendee -You mentioned that sponsors could request agreement like those of large universities. Does that mean it would come to our post office?
PS -They would come to you direct as they do in country, rather than going to Post Office. Overseas cards do have additional security steps that need to be agreed. The Home Office will need to ensure that premises and staff cleared to handle cards meet with security checks ahead of handing over.
When physically handing over the card employers/legal representatives would need to check the following;
• Check landing stamp on vignette
• Check photo matches person
• Check name and DOB the same
We plan to roll out this facility from June to universities ahead of autumn enrolments and allowing other bodies to register after June.
Attendee -This is a really positive move and one our clients have been concerned about.
Attendee -In relation to transfer of conditions, is there a timeline which the BRP must be collected by, so we know when to contact you if there are contract delays?
PS -If arrival is delayed by 3-4 weeks from the start of the 30 day vignette the card will come back to UKVI. There is a safety margin built into card retention at Post office for small delays. However if delays are significant, such as 3-4 weeks, the card may be destroyed and there is BRP replacement fee. This is why it is really important that where there are any contract delays or delays in travel you must contact the local post and we can arrange for the card to be held at the Post Office longer. The assumption is that if the delay is beyond 30 days of temp vignette then card will be destroyed.
Note: 'PS' is Phillip Smith, the Head of Immigration Checking and Enquiry Service at UKVI.
Note: The Operational Forum does not, to my knowledge publish a lot of information about themselves or what happens at their meetings. They are 'invitation only' meetings for lawyers with a practice area in UK immigration.
The information given in this answer is totally consistent with StrongBad's answer. +1 for him and he gets the bounty. This answer supports and verifies his answer.