Thanks for your further questions
You can then claim a % - you need to add up all habitable rooms - excluding bathroom and kitchen and hallway in main home.
Then say this is 6 rooms, then you can claim
1/6th if just the office is fully and wholly business use or 2/6ths if both the lock up and the office are used, and wholly business use.
Then when you have established this 1 or 2/6th amount, then you restrict it to reflect the actual time you spent there.
(And you can claim this against the heating and lighing, mortgage interest)
Note that as you then plan to claim a % of the moergage interest - then you do have to consider capital gains as and when you sell this property in the future, and also are indicating to the local council that business rates should be charged on this wholly business element of the property -
Let me know if you have any further questions - but it would be appreciated if you could, in the meantime , press accept ot rate the level of service I have provided.