Am I right in thinking inclusive SMS messages on pay monthly mobile plans exclude texting from overseas? Do they just apply a higher unit cost for these texts or are they excluded altogether? Any idea typically what the cost of an overseas text is? Vodafone if it makes any difference

Yes, you are correct.

If you get Passport added texts are 11p.

Also:
Text inclusive
Customers on price plans with at least 1200 minutes get an allowance of 10 standard text messages a day which can be used anywhere in our Europe zone. These are in addition to the UK text messages included in your price plan.

Text bundle
Pay Monthly customers on any other Vodafone price plan can buy an allowance of 10 standard text messages a day to be used anywhere in our Europe zone for just £10 a month.

http://www.vodafone.co.uk/personal/price-plans/managing-my-costs/travelling-abroad/ways-to-save/index.htm

Prices are on their web site.

most if not all providers will text you when your phone connects to the nearest network with a breakdown of call and text costs. it depends where in the world you are but I don’t think I’ve ever paid more than 50p for a text.

remember it’s free to recieve texts (many people think that its not for some reason) so tell your friends to txt instead of ring and leave you a voicemail which costs a FORTUNE!!