The OP is only 17, so it makes sense to take a long view.
So long as people s**t, shower, shave and shiver, there will be a need for plumbers and heating engineers.
At the moment, times are tough for some because of three factors:
1) Stupid media articles about plumbers earning unrealistic wages have driven up the number of entrants.
2) The collapse in housebuilding has driven lots of tradesmen into the domestic market
3) Changes in EU immigration rules has drawn in a lot of east europeans
If you view it from the perspective of a 17yr old, likely to be 40 - 50 years in the trade, all these are very short term factors, and will go away. Over 5 decades there will be some recessions, some boom times and some ordinary conditions.
I believe its more important to consider
a) Whether you are comfortable with the lifestyle - moving from job to job, periods of feast and famine etc
b) Whether you are prepared to put in 5 - 10 years to become a skilled tradesman
c) Whether you are naturally a practical person.
d) Whether you like working largely on your own.
If you can answer yes to all of these, then I wouldn't let the short term issues put you off. If you can't, then you should probably look in a different direction.