No, X and Y were the UK year identifiers, but they were only issued for 6 months each to use up the prefix letters before the current UK system was introduced in 2001. So X was for registrations from (uk date format did/m/yyyy) 01/09/2000 to 28/02/2001, and Y was for 01/03/2001 to 31/08/2001.
Apologies if you know this already; the UK used the prefix letter to indicate date of first registration of the car between 1983 and 2001, with registration years running from (UK format) 01/08/yyyy to 31/07/yyyy+1. Most letters were used, with I,O,U and Z omitted for clarity. The only special code used was Q, which was applied to home-built, rebuilt or imported vehicles where the age cannot be determined.
So the Peugeot was first registered in 2000, hence the X prefix. I bought it at 9,000 miles in 2001 and kept it until last year by which time it was showing 240,000 ish miles. Not bad for a petrol engine.
The Toyota I bought new and the SY13 shows the current UK system with SY indicating registration in the Inverness area, and 13 showing registration between March and August of 2013.