Is it OK to have underfloor heating as your primary heat source for a
room (I'm looking at installing it into my kitchen)?

Yes, it is fine as the primary heating, provided that the room is well
insulated. The output is limited, and can't heat a room on its own that is
poorly insulated.

If so, what type and output would I need to get? I'm only looking at
covering about 9 square metres.

Assuming you have wet central heating, the best method is to use PEX pipe
buried in the floor (concrete floor) or between the joists (wooden floor).
This is easier to retrofit to wooden floors, as the concrete will normally
need digging out to add insulation and pipework.

Christian.