I work in the air cargo industry and hopefully I can put your mind at rest a little. In the UK, there is a strict security regime for aircargo and it ensures that all cargo loaded on a passenger plane has been subjected to appropriate security checks prior to loading. It is true that these checks are not always 100% x-raying but can be another accepted method. Needless to say, all accepted methods are OK in terms of the safety and security of the airplane and you should not worry. The regime is too long winded for me to fully describe on this forum, but you could check out the Department for Transport website for further info (copy and paste this link into your browser):

http://www.dft.gov.uk/stellent/groups/dft_transsec/documents/page/dft_transsec_023330.hcsp

If you are travelling on a UK airline, they will ensure that their handling agents overseas comply with similar procedures for your return leg, so you shouldn't worry too much about that either....

Hope this helps!