... I see the Axminster super precision one is straight to within 17 microns over 600mm - is that accurate enough for you? I've just got the ordinary one - I mostly use it when straightening stuff out after welding (checking the effectiveness of the blows I make with the sledge hammer!). Graham.
And when it's released from the bed of the machine you'll most likely find it's not very straight at all.
Using hot or cold rolled material to make straight edges is not a good idea due to all the internal stresses that tend to relieve over time or with changes in temperature. Machining or cutting and edge is a sure fire way to change the stresses in a material.
The best straight edges are made from cast iron or granite for good reasons.
Having said all that if it's only for cylinder heads a real engineers straight edge is probably a bit overkill.
For cylinder head checks I'd go with the Axminster super precision straight edge with the beveled edge. This will be best for checking contact with a bright light shining from behind.
Just make sure you look after it. That sharp knife edge could easily get damaged and ruin the accuracy.