Given a local british datetime, convert to UTC - take off an hour in summer

ConvertLocalBritishTimeToUtc

public static class DateTimeExtensions
{
    private static readonly TimeZoneInfo TimeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById("GMT Standard Time");
    
    public static DateTime ConvertLocalBritishTimeToUtc(this DateTime dateTime)
    {
        var isSummer = TimeZoneInfo.IsDaylightSavingTime(dateTime);
        
        if (isSummer)
        {
            dateTime = dateTime.AddHours(-1);
        }
        
        return new DateTime(dateTime.Ticks, DateTimeKind.Utc);
    }
}

Updated the to explicitly state that the outgoing datetimekind is UTC.