At Cambridge railway station or bus station there are usually plenty of taxis waiting to take you the short distance to Fitzwilliam College, or wherever else you care to go. If you hire a car at the airport, then ask for a map. The quickest way to Cambridge will be via the M25 and M11 motorways. Some banks also open on Saturday mornings. Some car parking is available both at the Institute of Astronomy and the Cavendish Laboratory.
Permits are unnecessary.
The Weather The Cambridge summer is occasionally hot and sunny, often warm, regularly cool and frequently wet. Be prepared for anything, and expect some rain. Nights are usually cool.