UH is currently running
Apache 2
PHP 4.4.4 or PHP 5.2.4
MySQL 4.x (Can't remember)
Mcrypt and Sendmail are definatly suported, and I'm sure simplexml and DOM are also available, if they are not then support may be able to install them for you depending on the comlexity of it.
If you have specific questions then it might be worth you sending the support team an email via
UnitedSupport.co.uk - UnitedHosting Customer Support Site - Powered By Kayako SupportSuite, support desk is manned 24/7 whilst the forums are checked regularly you will probably get a faster response from the support desk.
PHP5 is not a problem to enable, just 2 extra lines in the top of your .htaccess file, and soon it will be the only PHP version available on the server, with PHP4 being removed completly
Good luck
Simon