How extensive are trade networks at this time? Can I buy amber from the Baltic, tin from Britain, silk from China, spices from India?
I’m not too sure about Chinese goods. But certainly you would be able to find goods from most of the Mediterranean and beyond in the Agora. Pericles even boasts of this in his famous Funeral Oration “to the Athenians the fruits of other countries are as familiar a luxury as those of our own”.
Attic (Athenian) pottery was a major export that has been found as far afield as France, Spain, and North Africa and we know that Solon the lawgiver introduced a system of weights and measures around 600 BC to allow Athens to compete and trade with other large trading centres. So trade and imports had been integral to the Athenian economy for over a century before the period you mentioned. By the mid 5th century BC the dominance of the Athenians in the Aegean gave them almost complete exclusive control of sea trade in the Greek world and Athenian currency and weights and measures were considered the standard in many other states.
To the East were the Phoenician states who were well known for their maritime trading colonies, as well as the Egyptians. Evidence for trade with these peoples exist well before the Classical Period so it would not be unusual to find luxury goods such as Tyrian purple in the marketplace. To the west were the Greek colonies in Italy which had a close interconnected trading system with mainland Greece, and further afield were the Carthaginians whom the Athenians would have likely traded with too. Playwrights such as the comedian Aristophanes frequently mention delicacies and luxury goods from other lands so it is likely these were commonplace enough in Athens in order for the Athenians to understand the jokes. Though more exotic imports were of course more expensive.
At the current time you mentioned (around 440 BC) the Athenians under Pericles had supposedly led an expedition to the Black Sea. Plutarch claims that Pericles demonstrated Athenian naval power to the cities along the coast, before settling colonists in Sinope and deposing the tyrant Timeileos. Modern historians have doubted the dating for this expedition, but if it did exist it may have been to secure Black Sea imports of grain, as later into the Peloponnesian War the Athenians relied on imports from trading partners in the Black Sea as well as the island of Euboea to support the population of Athens.
Trade was so vital to Athens that Piraeus, the main harbour, was fortified in the decades after the Persian Wars with a series of “Long Walls”, miles in length that connected the harbour to the city of Athens. One of the reasons why the Peloponnesian War dragged on for so long was because the Spartans could not cut off Athens’ naval imports through the Piraeus, and could not attack the fortifications.
As for Britain. Yes the Athenians would have traded, though most likely indirectly. Herodotus (3.115) calls it Cassiterides and mentions that “I do not have knowledge of the Tin Islands, where our tin is brought from... nor for all my diligence have I been able to learn from one who has seen it that there is a sea beyond Europe. All we know is that our tin and amber come from the most distant parts”
Hope this is helpful!
And interesting book on the subject (which you can search in google books)