For educational purposes, what can you expect to happen if you inadvertently infringe CAS? Didn't realise there were fines etc...
Like Gertrude has said, contacting the relevant authority ASAP with a meek apology generally does the trick, especially if no aircraft under their control had to be re-routed to avoid an unknown contact.
If they have to chase you up and/or submit a formal report, then it comes under the aegis of the CAA, who may take a different view if they believe your planning, execution or general airmanship was at serious fault.
Have a look a some of
these (caa.co.uk). The heavy fines are reserved for things like dodgy travel agents and shippers mislabelling dangerous goods but there are quite a few GA pilots getting their wrists slapped. Of course, they only show the successful prosecutions, so who knows what the conviction rate actually is.
This is a typical (and rather relevant to this discussion) example:
On 20 January 2011, an unknown aircraft was observed on radar within Stansted Transponder Mandatory Zone – 2. A passenger aircraft inbound to Stansted had to be vectored clear. The unknown aircraft was identified as G- CGJN. On 2 August 2011 at Harlow Magistrates Court, Mark Jefferies pleaded guilty to being the commander of G-CGJN which flew restricted airspace in contravention of Regulation 3(b) of the Air Navigation (Restriction of Flying) (London Stansted Airport)(No. 2) Regulations 2009. He was fined £1,000 and ordered to pay costs of £150.
That would have paid for a fully tricked out iPad Air 2, backup GPS and a 5-year subscription to SkyDemon plus a meal and a night out at the movies with money to spare...