IIRC the only UK phones that can roam there are 3G - for some reason they
never did GSM.

There is a pre-paid mobile service there (2G native to Japan), but because
of recent change in legislation it's very difficult for overseas visitors to
buy a handset and get the service activated, as you need local ID (such as
alien registration card - for long-term residents). There's nothing to stop
a resident or Japanese person to buy one and activate for you (though this
is strictly speaking illegal).
As stated, Japan don't have GSM but have 3G (WCDMA, European-standard is
used by two operators; another, au, use CDMA2000, more common in US). So if
you have a 3G phone and a sim from one of UK providers (all have a roaming
agreement with either NTT DoCoMo or Softbank - formerly Vodafone Japan, or
both), you can use your phone there with your UK number. But it won't be
cheap, with average rates around £1.50/min to call within Japan or UK and
£1/min to receive.
What I normally do is to take my 3G phone for incoming calls, and take
another unlocked 3G phone with international sim, TravelSIM, for outgoing
calls. The latter charges nothing for receiving calls (callers dial an
Estonian mobile number; only 9p/min through www.0844calls.co.uk) and
outgoing calls are around 0.45 euro/min (30p). You can get TravelSIM card by
mail order from www.travelsimshop.com.
3G coverage in Japan in generally good, though away from major cities it can
be patchy, and usually non-existent in rural or mountainous areas.

Alec