You should try brandywine or black cherry, they are amazing. And mortgage lifter is on my list of tomatoes this year (I try different varieties every year)
Mortage Lifter may very well work for you. I have tried to grow them a couple of times but they are a mid season and don't produce here because of the heat. I get a few of them but not enough to do more than experiment. Excellent tasting tomato although prone to early blight more so than others. Another large tomato you might want to try is called Jet Star, a large low acid tomato that is early to mid season.
Early blight, which is soil-borne, isn't usually much of a problem here for tomatoes, for two reasons:
Update, spent 20mins trying to find it in the UK without success.
Jet Star is an old hybrid and not very well known. Not many seed companies carry it.
Have you considered growing them UK-style, with only one stem?
Our growing season is from mid March through mid November. My last frost avg. date is April 1 . The problem with growing just about any large tomato isn't the length of the growing season it is the length of the setting season. Once the nighttime temps are above 76F they will not set fruit and this normally occurs late May to mid June so this greatly restricts me to mostly early season smaller tomatoes. This is why I experiment with so many different varieties, always trying to find one that will set fruit earlier. I can grow all of the large varieties such as the beef steaks and brandywines and have big gorgeous plants but little to no fruit simply because it gets too hot before they are mature enough to set. Cherokee Purple is the largest tomato I can successfully grow. I have grown many 1 1/2 - 2 pounds and always grow a minimum of 12 -15 pounds per plant, most years more. I have tried growing Brandywines, Goliaths, Beefsteaks etc every conceivable way. If it is a good year and I am lucky I might get at most 2 fruits per plant.
Well, since at 1 1/2-2lbs per fruit you're only talking 6-10 fruit. You have approx. 8 weeks from last frost date until end of fruit setting.
That is the secret. If you can make them set before the heat starts they will usually reach their normal full size. They will not abort but some may be somewhat undersized. Then there is the problem of sunscald. I must have a good cover of foliage when the daytime temps hover at 100+F.
Remember, even with the door open, a greenhouse (where most UK tomatoes are grown) on a sunny day can easily reach similar temps. We use greenhouse shading, or horticultural fleece, and the supermarkets in UK import a huge number of tomatoes from Spain, where temperatures are not so different to yours.