British pronunciation varies enormously depending on where you are in the British Isles. I'll give you a few tips about RP (Received Pronunciation), which is the most widely recognised 'neutral' accent, but please be aware that this is not the 'only' British pronunciation. Words and syllables ending a 's' or 'th' -or consonant clusters with these sounds - often have a long 'ah' sound. For example: pass, past, path, grass, grasp, glass, bath,, mask, task, cast, rather, father 'A' followed by 'r' also produces a long 'ah' sound: card, car, cart, jar, harm, Most other words and syllables written with 'a' have a short 'a' sound: cat, hat, map, ant, plan, man, act, blank, black, bad, I hope that helps a little. But be aware that there are some exceptions, and don't forget that these rules don't apply to all British people - English-speakers in Scotland or the north of England, for example, would not necessarily pronounce these words in this way.

"can't" is everyday language and "cannot" is more formal.