Would have thought that your GP could do a vital capacity test, if you asked. Blowing up a balloon is not how I'd choose to measure it, since the air inside is compressed...
However, Wikipedia is nearly always helpful:
It is possible to make a simplified spirometer of your own. Simply take two soda straws, and an aluminum can filled with water, allowing some airspace at the top. Place one straw deeply in the water, place the other one just above the water. Ensure that the straws are sealed in some way. Exhaling through one straw will displace water which can be measured as it exits the second straw. Multiply the displaced water volume by 10 for tidal volume and 100 for lung capacity.