Create a large fleet, then launch the British Expeditionary Force to colonize southern Arabia. Please note that you may have to break through the core Ptolemaic cities to get your fleet through the Red Sea Canal if you want to sail the entire way there. It will be exceedingly difficult to hold those areas, let alone start an expansion.

Keep in mind that you are vainly attempting to spread Celtic culture to the furthest reaches of the known world in a glorious adventure that will be continually sung by the bards throughout the ages. Slaughter your enemies and sacrifice them at the altar to your savage gods. Build religious monuments and start cults dedicated to the worship of British heroes. Construct great taverns and introduce the great art of binge drinking to the natives. And spread your biological seed in the sands of Arabia.