I had a bad weekend when I thought I'd get away with eating a broccoli while on RT, and boy, was I wrong. Barely came out the bathroom for 36 hours. My arse was so sore after that - I wasn't sure if it was the RT or the constant shitting and wiping at the time. Radiotherapists gave me Hydrogel cream - it's basically water in some gel thickener. That fixed it after a couple of days, so it was caused by the shitting/wiping, and not the RT which was still ongoing. They also gave me a tube of Instillagel (normally used for lubricating catheters, and contains antiseptic and anesthetic), but I didn't need to go that far. KY jelly might work if they can't give you anything, but certainly where I was done (Mount Vernon) they leapt into action instantly to fix any problems you might get during the RT.
After that, had to stop eating anything with fibre in it to regain control of bowels. No vegetables except peeled potatoes, no fruit except peeled apples, peeled pairs, and bananas, no brown bread/pasta/flour, no nuts, only low fibre breakfast cereals (corn flakes, crunchy nut corn flakes was strangely OK too, porridge without bran).