If you're swapping between street and track gear there's a limit to how much difference there can be depending on available track end length/chain length. With your street gear on try and set the chain length so the wheel is as far forward in the track ends as possible (but with enough movement that you can get the chain off without breaking it) so when you put the smaller sprocket for the track gear on it won't go off the end.
48-15 is a good starting point for Herne Hill but you'll want to go a little higher eventually, 50-15 or 48-14. For handlebars I'd recommend getting some standard road drops in the shape you prefer (compact or traditional) rather than specific track bars because you'll want to be able to ride on the tops comfortably when not on the track. If you're not used to riding in the drops for an hour +, as a rule of thumb you want your wrist to stay straight to help stop numbness, for me this means using classic curve bars with a flat bottom part tilted down slightly. It may take some experimentation to find the bars you like best so don't drop too much cash on the first ones you try.
If you're swapping between street and track gear there's a limit to how much difference there can be depending on available track end length/chain length. With your street gear on try and set the chain length so the wheel is as far forward in the track ends as possible (but with enough movement that you can get the chain off without breaking it) so when you put the smaller sprocket for the track gear on it won't go off the end.
48-15 is a good starting point for Herne Hill but you'll want to go a little higher eventually, 50-15 or 48-14. For handlebars I'd recommend getting some standard road drops in the shape you prefer (compact or traditional) rather than specific track bars because you'll want to be able to ride on the tops comfortably when not on the track. If you're not used to riding in the drops for an hour +, as a rule of thumb you want your wrist to stay straight to help stop numbness, for me this means using classic curve bars with a flat bottom part tilted down slightly. It may take some experimentation to find the bars you like best so don't drop too much cash on the first ones you try.