A more thermally efficient house - can I block up vents in my brickwork?
No, you cannot just block those air vents.

If you do you will cause condensation problems.

You need to investigate some cavity insulation systems - which would be more effective with your money compared to the triple glazing - think about the area of the walls and the areas of the windows. Whatever system of insulation is used, there must be some compensating air ventilation - good windows tend to have small air vents built into the top of them for exactly this.