Q:

Can I increase my highest gear ratio?

I have a modern bike (ridgeback meteor '13) and It currently has a 21 speed shimano chainset. 
I find that going down hills (which I do a lot) and on flats I need a higher gear. 
Can I do anything to get a higher gear ratio (such as a bigger large chainring) or am I already at the top?
Spec:
48/38/28 Chainrings
12-32 (7 Speed) Cassette:

A:

You probably can't fit a bigger front chainring on that bike due to the front derailleur and chain issues. They do make 11 toothed 7 speed cassettes which you can install (but I doubt you'll gain anything from it due to the second point I want to make on cadence), which will give you a bit higher gearing but the spacing between the cogs (i.e. the number of teeth) will be worse. Note that a higher gear ratio isn't always advisable, since you need to remain in control of the bike at speed as well (there isn't really engine braking, like there is on a car, in a derailleur system). Also, if you're commuting, you don't want to be going so fast that you get drenched in sweat when you get to your destination, so speed isn't everything. 
However, chances are you need to increase your cadence: http://sheldonbrown.com/gears/ says that at a 60 rpm cadence, you should be doing 19-20 mph in the 48t front, 12t back combo, which should be enough (the 700cx37 tires aren't on his calculator, but 35 and 38 are close enough). Your cadence might change if you're in an improper riding position (e.g. seat too high/low or weird handlebar positions). 
EDIT: The tire size on this bike should be 700c x 42. But the point still stands (the speeds should be slightly higher at the given cadence, since increasing the tire size also increases the gearing). 
You should also make sure your bike is tuned up (you may feel sluggish if your hubs are due for a repacking or your brakes are dragging on the rims or your tire pressure is too low, for example). 
If your cadence is already sufficiently high for some reason, time to get a road bike then (which will probably be lighter and more aerodynamic and have higher gearing (both at the bottom and top end)). Note that there will be a geometry change, even if you go with a flat bar road bike, probably, so if you choose to go this route, make sure to get fit. What you've got is a pretty heavy (~30 lbs, which is fine for its intended purpose) and cheap hybrid which is designed to be a commuter - it isn't really worth pumping money into it to improve it, but only to fix the things which are broken and add things useful for commuting, such as lights or switching to slick tires (which you presumably already have). 

A:

48/12 is actually a fairly large gear, especially since your bike has 700c wheels, the same as a road bike.  53/11 is the largest gear combination on a typical road racing bike, even on many road bikes ridden by elite racers at race speeds (e.g. 45-50km/h for an hour on time trials, or 60-70+ km/h for sprints).  That's only 20% higher than your highest gear.
I'm guessing you're riding at quite a low cadence: probably 55-65rpm.  90rpm is a typical target cadence for road riders. If you can get your cadence up even a little bit to 70rpm or more your problem should be solved.  I notice in your other comment you mention that you are spinning as fast as you can at 20-25mph.  If you're feeling like you're spinning out the top gear at that speed, I suspect that your pedal stroke is a little rough: if you have a smoother, more circular stroke, you should be able to spin at a much higher cadence without it feeling out of control.  This is easier to achieve if your feet are clipped in, but definitely possible to improve even with platform pedals.
In principle you could can get a roughly 10% increase in your top gear by either changing your front crankset or your rear cassette (assuming compatible components exist). Increasing cadence, however, doesn't require any component changes, will be more efficient and less tiring, and will be easier on your knees if you're doing a lot of riding.

