Modify submit page

Hi,

I use this plugin in inquiry mode and set the cart and submit pages. I found that there are some template files which can be modified. Is it possible to modify structure of submit page which can remain after plugin updates? For example my problem is that the my site operates only in the UK and want to remove all the other countries in delivery form except UK. Or is there any option to set default shipping country?

Thank you!

Best Regards,
Norbi