A mortgage in principle is an agreed figure from the lender that they would be willing to lend you. In order to do so they will gather basic information and perform a background credit search. A mortgage in principle is not a guarantee and a full application and assessment will have to be made before the lender can issue you with a mortgage offer. A mortgage in principle can last between 60-90 days depending on the lender. Because a credit search is needed, multiple decision in principles could have a negative effect on your credit score.
A mortgage offer is confirmation that the lender will provide you with a mortgage. This will be after fully assessing your circumstances and a full application is made alongside a valuation of the property. The amount of time it takes to get to this stage depends on the overall complexity of the application. In general, it will take between 18-40 days for the processing of the mortgage application and an offer to be produced.
Mortgage offers can last between 3-6 months, depending on the lender. Some will be valid starting from the date of application whilst others start from the date the mortgage offer was created. It is worth bearing this in mind before applying.