Currently you may not wish to use Holy Water that is available at the door…(due to flu season)
Nor is one required to cross self in entering and leaving (I assume it is the same in the UK)
One make use of Holy Water with the sign of the cross as a profession of Faith, as a sacramental that with prayer, the blessing of the Church etc can involve the forgiveness of venial sins and a reminder of Baptism. The Devil also hates holy water…hence it is used too in exorcisms and blessings etc.
I thought though it would interest you that:
In the early Church for the sign of the cross anytime they did a + on the forehead-- I still use this personally in addition to the common form now…
“Be the cross our seal, made with boldness by our fingers on our brow and in every thing; over the bread we eat and the cups we drink, in our comings and in goings; before our sleep, when we lie down and when we awake; when we are travelling, and when we are at rest”. St. Cyril of Jerusalem
And regarding the sign of the cross per se:
“Making the sign of the cross – as we will do during the blessing – means saying a visible and public “yes” to the One who died and rose for us, to God who in the humility and weakness of His love is the Almightly, stronger than all the power and intelligence of the world.”
-Pope Benedict XVI
Angelus
September 11, 2005
He also talks about it when he was Cardinal in the Spirit of the Liturgy…as a renewal of baptism…as a shield…etc…