Complex sentence: Upon RT's decision to create a separate UK cut-in channel the network allegedly attempted to place billboard ads in London promoting the channel and the ads were rejected as agencies felt they would be illegal under UK laws on political advertising.
Split into simpler sentences: Upon RT's decision to create a separate UK cut-in channel the network allegedly attempted to place billboard ads in London promoting the channel. The ads were rejected because agencies felt they would be illegal under UK laws on political advertising.