If you’re visiting the UK then I would recommend the following places to see Japanese swords, fittings and armour:

Victoria & Albert Museum
Cromwell Road
London SW7 2RL
Tel: +44 (0)20 7942 2000
http://www.vam.ac.uk
10am to 5.45pm daily (10am to 10pm Fridays)
Closed 24-26 December

British Museum
Great Russell Street
London WC1B 3DG
Tel: +44 20 7323 8299
http://www.britishmuseum.org
Open 10am to 5.30pm daily (10am to 8.30pm on Fridays)
The Museum is open daily except for 1 January and 24, 25, 26 December

Ashmolean Museum
Beaumont Street
Oxford OX1 2PH
(Opposite the Randolph Hotel)
Tel: +44 (0)1865 278002
http://www.ashmolean.org
Open: Tues – Sunday 10am – 5pm, Bank Holiday Monday 10am – 5pm
The Museum is closed 24–26 December

Pitt Rivers Museum
Parks Road
Oxford OX1 3PP
Tel: +44 1865 270927
http://www.prm.ox.ac.uk
10am – 4.30pm Tuesday to Sunday (and bank holiday Mondays)
12noon – 4.30 Monday

The Royal Armouries Museum
Armouries Drive (Clarence Dock)
Leeds, LS10 1LT
Tel: +44 (0)113 220 1999
http://www.royalarmouries.org
Open daily: 10am – 5pm
Closed 24-26 December
(You might also find some nihonto related items at the Tower of London, which is also part of the Royal Armouries)

Russell-Cotes Art Gallery and Museum
East Cliff Promenade
Bournemouth
Tel: +44 (0)1202 451858
http://www.russell-cotes.bournemouth.gov.uk
Open 10am – 5pm Tuesday – Sunday & Open Bank Holiday Mondays
(Closed Christmas and Good Friday)

Chiddingstone Castle
Hill Hoath Road, Chiddingstone, TN8 7AD, England
Tel: +44 (0)1892 870347
http://www.chiddingstonecastle.org.uk/
Open 11am – 5pm Sunday – Wednesday & Bank Holidays

Don Bayney Antiques
Greys Antique Centre
58 Davies Street
& 1-7 Davies Mews
London, W1K 5AB
Don’s shop is located downstairs (Shop A20-A21)
Tel: 020 7629 7034

The Lanes Armoury
26 Meeting House Lane
The Lanes
Brighton East Sussex BN1 1HB
England
Tel: +44 (0)1273321357
http://www.thelanesarmoury.co.uk