After a few tests I've found the following:
118 - doesn't connect
070 - doesn't connect
0843 - connects
0844 - connects
0845 - doesn't connect
0870 - doesn't connect
0871 - connects
0872 - connects
09 - doesn't connect
All 08 calls which do connect appear to be charged at 25ppm.
International calls only connect for me if I prefix with +, not when I prefix with 00.
Have you tried a range of 1p to 13p per minute Service Charge numbers on each of the 084 and 087 ranges to confirm the cut off isn't directly related to the level of Service Charge imposed?
Setting a call price of 25p per minute is a breach of Ofcom's new Clear Call Rates for Everyone regulations which require a consistent Access Charge for all 084, 087, 09 and 118 numbers that the network allows to be called.