Regarding the formal diet, you need to be on the exclusion diet for 8 weeks before attempting to reintroduce foods. This takes about one year to do properly. You will still experience unexplained flare ups. I would not suggest attempting the fodmap diet in the UK without speaking to a nutritionist first. You will need a go referral.
The AMT is the excess of the tentative minimum tax over the regular tax. Thus, the AMT is owed only if the tentative minimum tax is greater than the regular tax. The tentative minimum tax is figured separately from the regular tax. In general, compute the tentative minimum tax by:
If your income is above the AMT exemption levels (see table below), there are a different set of rules and rates for figuring your taxes. E.g. The AMT tax rates start at 26% and cap out at 28%. In contrast, regular tax rates start at 10% and cap out at 35%.