I guess it is the MCB that is tripping at switch on? MCBs trip because of over-current so it is a rather strange occcurance as the LED lights have a much lower current than the lights you have replaced.
I have seen this problem with a large number of halogen lights on one circuit as the in-rush current can trip the MCB, but never with LEDs. I rather think you may have some other problem.
To return to your query. Do you have a multimeter or two-probe volt tester? Before putting your fingers inside a consumer unit you must use safe isolation procedures to make sure the power really is off.
Read this guide
http://www.electricalsafetyfirst.org.uk/mediafile/100117573/Best-Practice-Guide-... especially the part from page 9 onwards. Take special care if you have solar panels or other auxilliary sources of supply!
Having isolated the consumer unit using the MAIN SWITCH, note that there will still be lethal voltages on the terminals of the switch itself.
The next issue will be to confirm that the EFLI (Zs) of the circuit is within the limits of a Type C breaker. Remember that
Type B - trips between 3 and 5 time full load current
Type C - trips between 5 and 10 times full load current
so the allowable EFLI for a C breaker will be about half of that specified for a type B breaker.
This table gives you the figures
http://espm.co.uk/Max%20Zs%20tables%2017th%20Ed.pdfNot so easy huh?