Both, actually. Using myself as an example: I have a little over 100 hours TT (75 PIC), done the aerobatics course and are now starting training for my first aerobatic competition. I have also been to the UK a grand total of two times (three legs as PIC), both times in a very-well equipped DA-40 TDI (dual GNS430, ADF, autopilot, mode S). If I were to have to do that in our Robin 2160, which we use for aerobatics, and which only has one VOR, no DME, no ADF, no autopilot and a GPS without moving map and a 5+ year old database, I would like to have a friend with me. Far enough away not to cause a hazard if your attention lapses for a dozen seconds, but close enough to see. The 1/3 of a mile that Rod1 mentions sounds good to me.
Yes, I know it would be legal and doable to take the Robin across alone but at the end of the day it is a minimally-equipped Day-VFR only aircraft. On a channel crossing, particularly the lack of DME, to back up dead reckoning, is something that worries me. Especially having seen the perfect-VMC-is-almost-like-IMC phenomenon. As Rod1 said, the friend provides a safety net that's pretty invaluable if something goes wrong. No better way to ensure a quick rescue than a plane circling overhead.
Having said that, don't worry about the exact scenario. It's not relevant. Assume, again, two pilots competent enough to do the trip on their own with spare mental capacity, two planes with almost identical cruise speeds, and a "loose formation" with enough separation to avoid hitting each other if somebodies attention lapses, but close enough to see. What's discussed in the pre-flight brief, how do you operate en-route, what do you tell ATC, etc.?
And once again, this is a hypothetical situation. At this point in time I have no firm plans. I'm here to learn about the risks and the rewards. If the risks are too great, I won't do it. But just telling me not to do it, without why, won't teach me anything. Yes, I understand that flying in a tight formation (and that is, as far as I'm concerned, anything close enough to give me the jitters) is risky. But flying approximately five minutes apart, same track, without a DME to give accurate position reports is risky too.