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Just keep your eyes open on this forum and the railway press; there's usually a small number of open days around the country each year and they do tend to get advertised and posted about several months in advance.
Various railway societies do depot tours. I am a member of the Inter City Railway Society. We have just done Long Marston, annually, we also go to Crewe Basford Hall and Eastleigh Works. Last year our AGM was at the East Lancs Railway, so had a tour of Baron Street depot. Also this year has seen members only visit to WCR at Carnforth.
The ICRS website is:-