London is always a great idea! With so many museums, historical sites, royal homes & gardens, and world-class shopping & dining, there's always plenty to do in this world capital. London is easily accessible from Naples Capodichino airport via non-stop flights into Gatwick and Stanstead airports on British Airways and EasyJet. Both airports are within easy commuter distance on non-stop train service to Victoria station (from Gatwick, 30 minutes) & Liverpool Street station (from Stanstead, approx. 30 minutes). Both stations are centrally located & connected to the overall London Tube system. However, be advised that on some major UK holidays (i.e. Boxing Day), Tube workers will sometimes strike, forcing you to actually get to those stations via taxi before getting on the airport express trains.
London at Christmas is an especially good idea, truly the stuff of Dicken's "A Christmas Carol." For a truly festive time on Christmas Eve, check out the Christmas Eve Carol Service at St. Paul's Cathedral (4pm) or various masses at Westminster Abbey, including a midnight mass. Reservations/tickets are not required, however, be sure to arrive early (as early as 3pm to St. Paul's and 9:30pm for midnight mass at the Abbey), in order to queue up. Lines can be very long and late arrivals are turned away once the churches are filled to capacity. If attending midnight mass at the Abbey, it is advisable to reserve a taxi ahead of time, lest you stand on a cold street corner trying to flag one down at 1:30am on Christmas Eve. Reserved taxis will wait just outside the Great West Door entrance. For Christmas Day dining options, be prepared to pay a premium at high-end restaurants (upwards of 100 pounds/person), but many smaller pubs offer Christmas Day dining for around 30 pounds/person as well. Most large businesses, shops, etc. (such as Harrods) are closed on Boxing Day (Dec. 26), but some stores and major shopping areas/centers (i.e. Regent Street, Savile Row, Oxford Circus, Sloane Square, etc.) are beginning to stay open on Boxing Day to offer post-Christmas sales.
A fantastic example of Georgian architecture, Bath is an excellent center for commerce, shopping & historical sight-seeing. Be sure to check out the Bath Abbey which contains one of the world's finest examples of fluted-vaulting within the nave. Adjacent to the Abbey are the ruins of the Roman public baths (yes, where the city derives its name), which is an impeccably curated museum & a must-see for visitors already familiar with Roman historic sites within Italy (contains an excellent giftshop, as well!). The shopping district of Bath surrounds the plaza containing the Abbey & Roman baths and is quite comprehensive with many British name brand shops & chain restaurants (i.e. Monsoon, Gieves & Hawkes, Conwall Pasty Co, etc.), but also lots of local pubs and specialty stores. For Jane Austen fans, Bath houses the Jane Austen Museum in a home where she once vacationed just blocks from the Bath Circus area (nicely presented with an excellent bookshop).
Situated in the Salisbury Plain of England (along with many other mystical, religious & neolithic sites), Stonehenge is approximately a 2-hour drive from London. While it's certainly the most famous of the sites in this part of the UK, it's not the only one in the area that bears mentioning. Nearby Glastonbury (home of the famous annual music festival & approx. 1 hour from the henge) boasts beautiful abbey ruins (from Henry VIII's dissolution of the abbeys) and sites pertaining to the legend of King Arthur. The Glastonbury Tor has been revered as the reputed site of "Avalon" for centuries and the summit can be reached via a lovely hike through pastoral fields to reach the bell tower ruin of St. Michael's. At the foot of the Tor lies the Chalice Well, which is purported to contain healing waters & is said to be where Joseph of Arimathea buried the chalice used by Christ at the Last Supper.
Also within easy driving distance of Stonehenge for more Arthurian-enthusiasts, is the beautiful town of Winchester. The Great Hall of Winchester Castle is the finest surviving example of medieval great halls & the only original portion of the castle that remains. It dates from the reign of William the Conqueror (1066-1087) and houses a replica of the famous Round Table, constructed and painted in the 13th century. Excellent high street shopping & a beautiful cathedral (burial place of Jane Austen) in Winchester, as well.
For those interested in British Naval history, do not miss Portsmouth, home to one of the most famous ships in history, the HMS Victory! A truly excellent experience with museum collections, memorabilia, one of the sails from the famous Battle of Trafalgar and plenty of Admiral Lord Nelson images (for the ladies, naturally!). Admission also includes a guided tour of the Victory by active-duty sailors in the Royal Navy.