I don't have the statistics on comparative casualty rates, but over 49,000 men of the British Royal Artillery were killed during the war. While not necessarily in the forward trench lines they faced their own set of dangers.

The greatest of these was counter-battery fire. Gun batteries were a huge threat to the enemy, and so were a priority target for their own artillery. Counter-battery fire became a growing danger to artillery crews as the war progressed and technology improved, especially with the advent of air reconnaissance. There's a good AskHistorians thread on this:

How effective was counter-battery fire

The idea of an artilleryman being far from the trenches is not necessarily accurate, though - Royal Artillery personnel were spread across the 'battle zone', from rear lines through to saps and observation posts out in front of the forward trench line.

Furthest back (not counting personnel in ammo dumps, depots etc.) were the largest calibre howitzers and siege batteries (The Royal Garrison Artillery in the British Army), which had tremendous range and destructive power - these are possibly the sort of guns that you are thinking of as being far back from the fighting front.

Much more numerous were the lighter, faster firing, horse-drawn field guns and medium howitzers of the Royal Field Artillery. These were deployed in immediate support of the front line and were very much in harm's way. These were reasonably mobile, but even more mobile was the Royal Horse Artillery, with even lighter guns and well practiced in fire-and-move tactics on the attack or retreat. In the appendix to the revised edition of his memoirs, With a Machine Gun to Cambrai, George Coppard publishes correspondence from J Skelton, who was an infantryman with the 35th Brigade:

Our next big do was when we were suddenly awakened and transported by motor bus up to Albert where absolute confusion reigned. Nobody appeared to know where Jerry was. Our horse artillery would gallop up in front of us, fire for a while, then gallop back. Then we would retire under heavy shrapnel from one ridge down the valley and up to the next ridge.

Further forward still were the medium and heavy trench mortar batteries, which were manned by men of the Royal Field Artillery (light mortars were manned by men drawn from the infantry) and positioned in gun pits within the trench line itself. Men serving these were as vulnerable to enemy action as the front line infantry, if not more so, as trench mortars were again high priority targets once they had been identified. For this reason they were unpopular with infantrymen in nearby trenches, as they would attract artillery fire onto their area. In Trench Warfare, 1914-1918: The Live And Let Live System, Tony Ashworth notes that mortar crews were informally known as 'suicide clubs' by the infantry.

Further forward still were the Forward Observation Officers and their men, who were in contact with batteries further back and had various roles such as registering targets and directing and adjusting fire from the guns. They needed to be in positions which afforded a view of the enemy, which might be on high ground in relative safety, but would often be in the forward trenches with the infantry, or even observation posts out in front of the line - they would of course be sharing all the risks of the front line soldier.