It will depend on a number of factors:
1) If it is from outside your country you will almost definatley have to sign for it.
2) If it is standard 1st, 2nd class etc you will not have to sign for it.
3) If it is using a method such as Recorded Delivery, Special Delivery, Express Post etc you will definatley have to sign for it.
4) Just to confuse you sometimes when the package is letterbox/postbox size Royal Mail will just post it if your not in as oppose to making you sign/collect it later when using the methods in 3) but if international will always keep ahold of the package untill you can provide some ID.
Hope this helps