It depends on what you intend to trade ie Forex, Indices or individual shares and over what time scale, intraday requires less margin than trading futures
If you are looking to trade indices futures then £50k would be a reasonable pot and would allow you to trade with little pressure. If you havn't traded before I would sugest looking for say £500 per month and increasing as you develop your money management and trading skills.
If your looking to scalp trade shares then Mr Charts is your man but you had better be sat down