It's probably a government scheme to pay for the Royal family.
For real, wtf?  Do lotteries work differently in the EU than the US?  If I buy a lottery ticket nobody would know that I won unless I say, "Hey . . . look.  I won!"  Why would you get this letter and think it was real if, for example, you hadn't bought a lottery ticket or, for example, if you didn't have a winning lottery ticket in your possession?  Or is it just something into which every taxpayer is automatically entered?