Is minicabit a reliable option to use to get between airports?
it's simply a price comparison site
you could
use it,
find a taxi firm yourself (e.g. http://www.blackberrycars.com/ )
or take the regular coach http://www.nationalexpress.com/airports.aspx
Unless pressed for time, use the bus or coach in British terms. Not familiar with minicabit.
Minicabit appears to be a general booking site for minicabs, so you are dependent on the quality of the individual firm they connect you with. Personally I would go with the National Express coach - it is non-stop between the two airports and won't be much slower than a minicab.