It is possible to use eSignal and IB together. See http://individuals.interactivebrokers.com/en/software/tws/usersguidebook/configuretws/esignal_market_data.htm
There is no automatic exchange. You are borrowing USD. Interest rate charges are due if you keep positions overnight. Alternatively, you can buy USD on IDEAL or IDEALPRO.
It is a tradeoff. You can keep pounds and pay interest on USD that you are borrowing from IB to cover margin. No interest will be charged if you are daytrading only. You can buy USD on IDEAL and do not pay interest. You will depend from currency exchange rates in this case.
Unbundled is almost always better if you do not keep positions overnight. If you keep positions, then go bundled. In any case, it will not make a big difference with a small account.