I have just got some weights they are.
2 X 1.5kg. 2 x 3kg and 2 X 5kg

I will use these to do my shoulders, pecs, biceps and triceps. I am 15. Does this sound like a good weight for strength for punching and general strength.

Ill probably use the 1.5s for heavy hand punching. The others Ill use for chest and biceps. They were only £13!! for the lot.
My dad bought them for us to use.

For hand speed and maybe some shoulders, these will be good. If you are 15, your ahead of the game, but listen up: Don't go any heavier than these and maybe a little heavier until you turn about 17 or 18. You don't want to start ripping on your muscle fiber while its still growing at naturally accelerated speed. Use these weights to perfect your form for lifting and such, and in a couple of years, get heavy with it. Stay ahead of the game, way to be motivated :)