Direct Access is changing at some point - they are raising the age limit to 25, check with your local Bike School to see when this is coming in.
Anyways...
Bikewise a standard is best all round, preferably one with a flat bench type seat that allows you to choose your riding position.
I am on a friends CB500 at the moment and it is enough power to do everything and then some. I choose to buy a 250 because its just too easy to speed on the 500.
I put some serious mileage in on my CG125 when I had it and I totally loved it, however at 65mph it's very buzy as it is running near the top of its abilities, after an hour or so I'd find my fingers would be numb. For any kind of distance work I'd suggest 250 as the minimum.
As for back comfort, sitting on one is the only way to spend time on one. During my lessons I was on an ER-5 which totally cramped my legs and I initially found to have an uncomfortable riding position. I did adjust but the first few hours were a misery because it's riding position meant I was leaning on the tank with my groin. Eyewatering to say the least.
I love the look of standard/naked bikes but after doing a lot of distance work on my friends unfaired CB500 I found that the windblast to my chest and head could be uncomfortable. The good news is that Honda released a version here with a fairing, the CB500S, which would take care of that and still looks pretty good.
It may not be an issue for you but insurance wise I've been charged £148 for TP,F&T over the year on the 250 with 2 years no claims.
That CB500 is nice. I wonder what it would take to get one in the U. S. You lucky Europeans, I'm frigging jealous. The only street legal Hondas with a standard seating position available in the U. S. today are a dual purpose 650 and the Nighthawk 250, both of which are way overpriced.
If at first you don't succeed, skydiving isn't for you.
The GPX250 (EX250 in US) looks to be exactly what I'm looking for in a beginner bike. The only problem is that they seem to be fairly hard to come across! Are there any other bikes, perhaps from other manufacturers, which would be similr to the GPX250?
Google is not very helpful when it comes to finding beginner sports-tourers
I live in Lancashire but I'm frequently in the Stafford area. I'm planning to head to a few showrooms and try sitting on some bikes before I actually make a decision.
With traffic being what it is in England and you're wanting to be in the saddle for 4 hours at a time, It sounds as if you'll be using the bike on trips versus commuting. Almost all of the big bikes are great for road trips on winding roads. The Honda VTX is considered to be the best handling V-Twin with all of the other bikes a close second (yes even Harley handles good) Touring bikes offer the best of both worlds for you. they have a comfortable riding position with your feet directly below and a slight forward lean for your torso. And they handle well in traffic. They are lighter than V-Twins or Cruisers and so are more nimble in city traffic.
By the way, could you come over here and lobby our government to allow bikes to split traffic and go to the front of the line at a light.
