What tends to happen:
- Spreads at 08:00 can be wider than on a normal morning.
- The first move sometimes reverses, as orders that built up over the holiday get processed.
- Other markets traded the day before, so GBP pairs have to catch up with news.
How to check whether it's useful for you: compare spreads at 08:00 and the first-hour range on post-holiday mornings with normal Tuesdays. There aren't many such mornings a year, so collect data from several years.
The cost of the rule is small: maybe a missed trade a few times a year. The benefit is avoiding one of the less predictable openings.
Does anyone trade differently after UK holidays?