Beat the spread
If you want to get into spread betting which is basically not much different than gambling, you need to have sufficient cash reserves to make sure that you don't get wiped out by a losing streak, and that you do not give the house an excessive advantage.
1. Losing Streaks . When you gamble you are likely to encounter losing streaks; this is when you lose several times in a row. If you are to remain in the game you just cannot risk running out of money. This requires you to think carefully about the extent of consecutive losses you are likely to suffer. If you make just one spreadbet in a trading day and you have about a 55% possibility of getting it right, then you also have about a 70% chance of getting 5 losses in a row in your first year. This means that you must ensure that you bet sufficiently small that you are able to absorb 5 consecutive losses (i.e. won't wipe you out). Most spread betters advise that you only risk a maximum of 2% of your available funds in any one spread bet. That is, the maximum share buy you should make should not exceed 2% of your trading capital.
2. Don't hand the house your winnings. Commission and bid-offer spreads (difference between buy and sell prices) add up and eat into your profits. Keep in mind that just like a casino the house always has an edge so the house always wins in the long run. Even the very best traders get it right only slightly better than the toss of a coin and in order to make money, your edge has to be bigger than the house's advantage. In practice you are unlikely to be a top trader so you need to be more aggressive. For one thing you should aim to keep the house commissions (i.e. mainly bid-offer spreads and daily financing charges) to less than 2% of your target profit. This means that if you pay £10 to buy and another GBP10 to sell you should expect a profit of at least 1000 before you enter the trade.
Rule 2 implies that you should have a target profit of about £1000 per each trade. Since most equities move less than 10% at a time, this in turn requires spread bets equivalent to a market exposure of £10,000 in the underlying.
Combine that to rule 1 which suggests having at least 33 times your maximum buy in cash. This would translate into a minimum starting capital of about £300K. This is way too much for most people and in practice you can limit losses by using stop loss orders which can make it doable to spread bet on a daily basis with a much smaller account. However, a good rule of thumb is that you need to be well capitalised to stand a good chance of being able to make trading a daily profitable activity.