Founder of Velocis, a leading web & mobile startup studio in Dallas, TX best known for incubating Gig Wage and Student Success Agency.
This sounds like a "Convertible Note" which is fairly common, at least in the US. Here is a good definition: http://startuplawyer.com/startup-law-glossary/convertible-note
I'm not sure how these are typically structured in the UK so you'll definitely want to speak to a local lawyer about it.
The short answer here is that this is very situational stuff and not necessarily something you can have completely planned out in advance.
If you've already incorporated (c-corp) then you'll have authorized shares and issued shares (see http://startuplawyer.com/incorporation/how-many-shares-should-be-issued-to-founders-at-incorporation)
Any additional grants would then be dilutive.
In most cases, you'll want to create a 10% option pool for future employees. Co-founders that you would bring on will typically want common stock.
Advisors or consultants that are helping out for equity would typically receive option grants that come out of the 10% pool you set up.
Long story short, depending on your situation and needs there are A LOT of ways this could go down. I would recommend reading up more about these issues at http://startuplawyer.com/ (author is a personal friend and his site is packed with great info)
Hope this helps!