If you can't find your paperwork try
http://www.halifax.co.uk/ click on ISA the 'find out more', that gives the terms of present isas. I'm sure you could find the older ones somewhere on the site too. This is what it says
Closing your account early will result in the loss of an amount equivalent to:
1 year term – 90 days’ tax free interest
18 month term - 135 days' tax free interest
2 year term – 180 days’ tax free interest
3 year term – 270 days’ tax free interest
4 year term – 320 days’ tax free interest
5 year term – 365 days’ tax free interest
So you have to know your interest rates to work it out and it may depend when you opened it.