It depends on the insulin that you are taking.:
People on this thread are using different types of insulin.
Sarah 69 uses a basal/bolus regime so missing a meal and injection should be fine.
A basal bolus regime is when you take a long/medium term insulin (basal) to cover the release of glucose from the liver and another faster acting insulin (bolus) for meals ie you would normally take 4-5 injections a day.
As long as you continue to take the basal then you should be able to miss a meal (and take no rapid insulin for it)
If you use a mixed insulin (as Sameer does) then it's more difficult . This type of insulin has the faster acting insulin for meals and the basal insulin mixed together in one pen. You can't split them. People on this regime normally inject 2-3 times a day.
If you inject the insulin and don't eat then the rapid/fast insulin in the pen may cause you to go hypo. If you don't eat and also don't inject then there will be no basal insulin to cover the glucose from the liver so your glucose levels may rise.
You really need to eat regularly on this type of insulin.
There are a few people who may just take a basal insulin (often lantus or insulatard) this is a background insulin and is just designed to cover your glucose from the liver . You would probably take it once a day and no insulin with meals in any case. Again you should always to continue to take this background insulin.
There is a list of different insulins and there types here:
http://www.diabetes.co.uk/insulin-in-the-uk.html
If you aren't sure what type of insulin you are using and how to use it you really should contact your nurse/doctor for some advice.