Accounts with FXCM UK are governed by ESMA regulations which requires negative balance protection on an account level basis. One of the the ESMA rules enacted this year was for negative balance protection limiting the overall losses for retail clients.
In other regions such as FXCM Australia the trading agreement specifies circumstances in which the negative balance will be waived by FXCM. You can read more in point 27 of the terms of business.
As a general rule of trading, managing the amount of leverage you use in your account is one way you can protect your own account from going into a debit balance. Leverage can magnify both profits and losses and would be the likely reason you would end up with a debit balance. The more leverage being used, the greater the risk. For example, if you had a long position in GBP/USD today and leveraged your account 10x , then the 1% drop would be magnified into a 10% loss of your account. If you were leveraged 100x then a 1% drop could be magnified into a 100% loss on your account. Any more than that and you could potentially go into a debit balance.
73.62% of retail investor accounts lose money when trading CFDs with this provider. You should consider whether you can afford to take the high risk of losing your money.