Shulhan Arukh, Yoreh Deah 268:1, says that if the proselyte is mutilated and therefore cannot be circumcised,"the[lack of] circumcision does not prevent his conversion, and it is enough if he takes the ritual bath." It is also a fact nowadays that most male infants, whether Jew or Christian, are circumcised by the obstetrician; therefore there are very few actually uncircumcised, or at least less of them, among would-be converts. It might be a worthwhile decision on the part of the English Reformers not to insist upon taking"the drop of blood of the covenant” if a convert is already circumcised(cf. the discussion of Asher b. Jehiel to the passage in Yevamot). If the English Reform movement decides to give up the requirement of circumcision, the fact that the Bible and the Mishnah have no such clear requirement, and the fact that this has been the practice of American Reform almost from the beginning, might aid them to the decision. If they wish to insist upon the requirement, then perhaps they will waive the requirement of taking the drop of blood from one already circumcised.
ADDENDUM
Since writing the above, I have come across an interesting discussion of the question in the responsa Hazon La-moed by Mordecai Dov Eidelberg, who was rabbi in Nickolayev, Russia . The book was printed in Bialystok in 1923. The problem that confronted him was this: A Russian officer was converted to Judaism by a well-known rabbi. The officer, however, was not circumcised at the conversion because he was not well at the time. He promised to be circumcised at a later date when he would be restored to health. The rabbi who converted him insisted that under these circumstances the officer was a full proselyte even though he was not yet circumcised.