The legal definition of Careless Driving is not prescriptive. In general it is taken to mean driving “where the standard falls below that expected of a careful and competent driver.” As a result the question posed by Dot (How was your driving careless?) is not easy to answer.
It could be argued that anybody losing control of their vehicle in the way you did is guilty of the offence. After all, careful and competent drivers should not lose control of their vehicle. However, there are circumstances where hazards present themselves (such as you describe) which could not have reasonably be foreseen, and so a valid defence is available.
Unfortunately the only way to test this out is to plead not guilty in court. Magistrates will then decide whether your driving fell below the required standard and they will base their decision on evidence provided by the prosecution and by you.
The maximum sentence for this offence is a fine of £5,000 (not £2,500). However, from your description this allegation seems to be at the lowest end of seriousness and magistrates’ sentencing guidelines suggest a “starting point” of a fine of half a week’s net income and the imposition of three penalty points. This is for a first time offender pleading Not Guilty and having been found guilty at trial. In addition you would be asked to pay a £15 Victim Surcharge and costs which would probably be around £500. If you decide to plead guilty the fine will be reduced by a third and the costs will be dramatically reduced to about £60. The points and Victim Surcharge will remain the same.
Before deciding whether to plead not guilty you would be wise to seek advice (unless you are prepared to gamble the additional fines and costs against the chance of an acquittal).