UK GAS-Prompt prices firm in near balanced market

* UK gas system 1 mcm long

* Britannia gas field annual maintenance begins

LONDON, July 28 (Reuters) - British wholesale gas prices for prompt delivery were firm on Tuesday as cooler weather supported demand in a near balanced system.

Gas for immediate delivery was up 0.52 pence to 42.80 pence per therm at 0805 GMT.

National Grid (LSE: NG.L - news) data showed the system 1 million cubic metres (mcm) long, with demand forecast at 187 mcm and supply at 188 mcm.

Demand was well above seasonal norms of 151 mcm, aided by cool weather, with Britain's Met Office forecasting temperatures to remain "unsettled and cool" over the coming five days.

The forecast for Wednesday was for average daytime temperatures of 15 to 17 degrees Celsius.

"Weather models for the UK point to temperatures remaining below normal for the time of year, only starting to return to the mean at the weekend," analysts at Energy Aspects said in a report.

Temperatures are also expected to remain below normal on the Continent, it added.

Annual maintenance at ConocoPhillips (NYSE: COP - news) ' Britannia gas field began on Tuesday and is expected to last for just under a month, decreasing flows from the UK's Continental Shelf.

Gas for winter 2015 delivery edged down 0.19 pence to 46.31 pence per therm.

Dutch July gas prices at the TTF hub were 2 cents lower at 20.48 euros per megawatt-hour.

Europe's benchmark carbon price was 4 cents lower at 7.99 euros per tonne on ICE Futures Europe. (Reporting by Sarah McFarlane; editing by Jason Neely)