An angel investor is an individual who invests their own money into an early-stage startup, usually in exchange for equity.
Angel investors are solo, whereas a VC fund invests money it has raised from other people and has a mandate to follow. An angel is spending their own money, so the decision to deploy or not lands with them.
Most angels have built or run companies before, and many still go alongside being an angel. On top of the money, they often bring specialised sector knowledge, introductions and time, though how much of that you get varies a lot from one angel to the next.
In the UK, angels generally need to self-certify as a high net worth or sophisticated investor before they can be shown certain early-stage opportunities. That is a regulatory requirement, not a description of the person.
Same type of investor, three quite different processes to get through.
Most UK angels investing at this stage will ask whether you qualify for SEIS or EIS, because the relief changes the maths on the investment.
Advance assurance is the confirmation from HMRC that your company is likely to qualify. Angels often ask to see it before committing.