Non Anglican Marriages

The registers of many Nonconformist churches only recorded baptisms and burials, although a few did include marriages. However, perhaps the main point you should be aware of is the fact that between 1754 and 1837, marriages that weren't conducted by the Church of England, Quakers or Jews weren't illegal as such - they simply weren't valid.

Obviously, that would be the main reason no marriages were generally recorded (and probably relatively few conducted). However, from records it does appear that some among the faithful did marry within their religion, which makes this an avenue worth pursuing, although in all likelihood you'll come to a roaring dead end.

Quakers and Jews

The biggest change for both these denominations came in 1837, when they moved to a printed marriage form that gave the same information as found in civil marriage registers. However, the Quakers had long since had a custom of all attendees at the wedding signing and witnesses, and continued it on their form (although there was no statement of their relationship to the bride and groom).

Catholics

Formal witnesses are also named in Roman Catholic registers, although pitifully few survive, in large part due to the persecution of Catholics in England for many years. That said, post-1850 you might be able to find marriage transcripts in the hands of the relevant diocesan authority.

Alternative Sources

Another good source for notices of marriage, especially in the 18th century among the middle and upper class, is newspaper notices, the forerunners of today's marriage notices. These can be remarkably informative, with not only names of bride and groom and the date and place of the marriage, but also parents' names, and in some cases estimates of the amount of the dowry! Perhaps even more remarkable (at least to modern eyes) are the notices of elopement, which also list the bride's maiden name.

If you're dealing with a family that possesses a coat of arms, they can offer great clues as to marriage. The coats of arms of a wife would be incorporated, and from that you can discover the family. You can even find the evidence of a remarriage in the "hatchment," a diamond shape used by heralds. A wife's crest was only carried on in her husband's crest is she was, or became, the heiress to a family with its own crest. Then it would continue into the next generation.

Married Abroad

When you find "married abroad" in a register, don't necessarily assume it means overseas. The phrase was commonly used to simply mean married in another parish. However, should you suspect the meaning to be literal, probably your first port of call on research should be the International Genealogical Index.

Problems With Registers

The biggest problem, then as later, was spelling. If the name of the bride or groom was written down wrongly, you might be able to check the actual signatures. However, as illiteracy was high, many just made their marks. But it's still worth looking if you believe the certificate should be there.