If sent by regular post there is a chance it may not be stopped. If declared etc. then it will be stopped and the postman will call and leave you a notice to come and collect with a level of duty marked on it.
For alcohol you must pay the excise duty (currently £22.64 per litre pure alcohol in the UK), this is added to the declared value and then VAT @15% is added to that:http://www.hmrc.gov.uk/budget2009/bn86.pdf
So a 750ml bottle at 40% = £6.792
+ declared value of $100 = £60
+ 15% VAT = £16.81 duty + VAT payable on this example