I think there are some varnishes specially designed for outdoor use. And some of them have UV inhibitors to slow down the sun’s effects. Another idea might be teak oil, but I don’t think that will help with the graying.
If it is for someone else, I would try to convince them of the merits of letting it go grey with minimal preservation. That is after all the main benefit of using teak.
All exterior finishes need regular maintenance to remain at their best, there’s no escaping it, and even more so on a bench which will be subject to mechanical wear and the full blast of the sunlight. Personally I am biased against exterior varnishes because of the time involved in maintenance, however if it is possible to remove the wood from the metal frame when it needs re-finishing varnish becomes much more viable.