Hi Caroline Baldry,
EIM32366, advises "An employee who holds a travelling appointment can deduct all of their business travelling expenses as travel in the performance of the duties of the employment, even where the journey starts from home".
There are other factors that have to be considered, so please read through EIM32366, to confirm that you meed the criteria for claiming expenses from home to offsite meeting.
Please refer to EIM32366 here:
EIM32366 - Travel expenses: travel in the performance of the duties: travelling appointments
Thank you.