The AC filter head isn’t Jaguar specific, it was the standard of the automotive industry for 20 years or so. The filter that it was intended for is GF124, which looks like this:
Fuel enters through the center, passes through the cellulose material, and passes out through the base. There is a seat on the filter head that accommodates the rim, and the seal is perfected by the gasket, which slips over the entire filter. The modern replacement paper filter, available from Wix, Baldwin, Fram, and others, looks like this:
As with the original Delco filter, the rim fits to the seat and the gasket fits over the entire filter. I think the gaskets sold as separate items by the usuals is an improvement over the gaskets that come with the filters. The filter itself is superior to the original cellulose filter for several reasons. First, the paper has far more surface area. Second, the center seals by compressing the carboard base over the spigot. This is much better than the gasketed seal of the original,as the spigot has very little landing area for a gasket.
Next, reversing the flow is very easy with these filter heads. GM did so on a couple of applications where the engine compartment required the filter to be “on the other side”. In this case, the fuel enters through the rim and flows out the center. The filter for these applications was GF149, which looked like this:
Pretty clever, right? There are modern versions of this as well:
If you use use this filter in place of the GF124 without reversing flow, then dirt will be trapped on the inside. There would also be a tendency for the filter to be pushed out of position by the flow. Which brings us to the filter sold by the usuals:
It’s like the GF149, but since the flange has been omitted, the filter is held in place with a spring. it’s really not a great solution. The spring makes installation cumbersome. Dirt is trapped inside, rather than outside where it can be seen. The one thing I like is the nice, wide gasket, but this can be purchased separately and used with a more appropriate filter.
Finally, adding an inline filter. There are any number of generic 5/16" filters that can be plumbed into the E-Type system, and it’s not a bad idea. If you can tolerate the sight, adding one between the AC filter head and the carbs would work best. Or adding one between the SU pump’s outlet and the bulkhed banjo would work. The one thing you should never, ever do is place a filter between the pump and the tank. The tank screen is sufficient protection for the pump. The reason why is that there is a chance of vapor lock between the pump and the tank, so you should never do anything which might lower pressure there. The proof of that pudding would be that on a very hot day, the pump could lose prime and you would have to wait for things to cool down before you could get it pumping again.
I thnk that covers the subject pretty well.