You will not be able to use Lightspeed in combination with IB as Lightspeed is an etrade platform. When I contacted etrade they told me that you will need to fund an account with $30,000 to use Lightspeed and they prefer traders who trade 10 to 15 times a day. I have found the whole issue of converting GBP to USD (with the intention of using a platform such as Lightspeed) an absolute pain and for the moment decided not to do so and instead use IB.
I think the answer to your specific need is all down to what suits you. For me I have used IB as a broker and as a datafeed with Tradestation 2000 and it was as reliable as any datafeed I have used. That said I have decided to use esignal for realtime charts and level II data with the special interface (provided by esignal) that allows direct trading with IB.
If you do use esignal and have an IB account, you get a subscription discount so make sure you get it if you decide to go down that route.
I should also say that I will be evaluating how effective my trading is using the esignal / IB route and if I feel that it is not quite what I need then I will be back to seeing the best way to convert GBP to USD and selecting a different Level II platform.
I have called IB specifically about this as I wanted to change all my fund into dollars and then trade in dollars. I was told that I would need to set up an entirely new account in dollars to be able to do this as my current account is in pounds. It may be possible to do it in a different way but if it is it is not as straight forward as I would like it and rather than face any problems I would prefer to just set up a dollar account.
IB won't let you change the base currency of an account. So if it's £ now you can't change to $. However, if you want to trade $ listed futures or stocks you just do it. They automatically create a position in $ which covers your position. It has a margin requirement, and you will pay interest, but it is effectively seamless. I have a £ IB account but I only ever trade in euros at present, there's no problem. If you want to move winnings to £, or cover losses you just buy or sell them.
If you are only ever going to trade in $ then a $ base account may be more effective, but you will have to cover exchange issues when you fund the account, or withdraw funds.