Once you have cleaned and lubed ( with the Gasket Cement ) it should not take much pressure... a piece of hardwood will work fine... I think they are made so that the outer lip of the plug is supposed to be even with the outside of the block... so you can still use a socket to start it straight ... but then use something flat to just flush it with the block.
This url shows the type of tool... not sophisticated...
but the nub in the middle..if you were making your own.. should be almost as wide as the ' bottom ' of the plug...so as to keep it straight going in....
http://www.circletrack.com/enginetech/ctrp_0811_freeze_plugs/installation.html
http://www.store.partsdinosaur.com/page57.html
http://www.jag-lovers.org/xk-lovers/library/freeze_plugs.html
That third one is interesting ... talks about an XK 120... I had an XK140 pretty much the same deal.... it had a freeze plug at the back of the head maybe 3/4 inch from the firewall ...and mine was leaking... instead of sealing it up with Jbweld and just be sure I was cold temperature protected.. I took the engine out to get to it..... that was a cast iron 6 cylinder with double overhead cams... never got it back into the car.... this was during college....had few tools, girlfriend, etc....