With both a History degree and being ex-UK military, I feel I can risk replying to this without it being deleted; although the post might get deleted for breaching the 20 year rule anyway.

Basically, every soldier in the British military has multiple uniforms. One of these uniforms is called the No.2 uniform (Google it) and is worn for formal occasions like parades. The units that guard Buckingham Palace and elsewhere are the Coldstream Guards and Scots Guards amongst others, and occasionally visiting Commonwealth units. Although the "traditional uniforms" they are wearing appear very anachronistic, they are actually just one of their current uniforms. The weapons that they carry are also their current weapons. It would make no sense for those units to keep and maintain "traditional rifles" as the processes that the UK military has in place to store, secure, maintain, and test these weapons would require significant extra upkeep costs. The rifles will be assigned to the soldier using it and kept in the armoury when not in use - the weapon 'belongs' to the soldier who is holding it.

Furthermore, it might be worth reminding people that those soldiers you see on guard outside the palace and other tourist attractions are currently serving and on duty. They are not just there for ornamentation and are doing their jobs - treat them how you would like to be treated at work.