I did convert my ATP license and I can tell you it is a painful process. There are different ways of converting depending on your level of experience. I would highly recommend you going for the UK license.

First of all, you need a Class 1 medical test. The first one has to be done in UK. Period. After that you are good and can renew it more or less easily. This medical test is very elaborious and expensive. I think today it cost around $650 with the exchange rate. Yes you heard me. I paid for the renewal $400 in FL. It is essentially a military medical exam and takes about 5 hours to complete. I was tested on things I don't even know what it was for. You can find some info at Initial JAR Class 1 (Professional Pilot) Medical Examination | Medical | Safety Regulation

The next step will depend on your experience. You have three options. Option # 1: If you have at least 500 hours on a multi crew jet + hold a type rating (second in command type is acceptable), you can right away take the 14 written tests and your skill test in a simulator at Flight Safety or other providers approved by CAA.

Option #2: You have at least 3,000 hours on a jet weighing more than 30,000 kg (x2.202 for pounds) on scheduled international routes (that's the killer) and of which 1,500 hours must be PIC, then you can receive a UK license valid for 18 months. The time restrictions is lifted once you pass 2 exams (instead of 14); Air Law and Human Performance.

Option 3# You have not jet transport experience and you will need to take those 14 exams. You can after take your commercial in US but the instrument rating has to be done in Europe. Officials want you to have European airspace experience. Before taking the skill tests you will need endorsements. Once you get the "hours", this commercial is going to become an ATP.

Option#2 is rarely used and I believe if you are flying a ERJ you are out. The only option you have left is #1.

Now, those 14 exams are not easy. The questions are very picky and the answers very close. You will need to take this study seriously and you will quickly realize that your experience won't necessarily help you. You will learn a lot of things you probably never seen like polar navigation. One memorable question I will never forget was: What is the lapse rate in a standard atmosphere? (a) 1C/1000 ft (b) 2C/1000 ft (c) 1.98C/1000 ft (d) 4C/1000 ft. The answer is obviously "c". Don't start screaming (it was my first reaction) and complaining... it's a lost battle. That's how picky they are in their answers.

There are however some companies (like Gleim) that offers an unofficial database of questions gathered over the years by students. This database is very useful but there are a lot of mistakes. www.aviationexam.com is one of them. The database includes about 15,000 questions. By the way, I highly recommend the books from Oxford Aviation/Jeppesen. it will cost you about 1,000$.

The bottom line is this all process will take time. If you are thinking of getting your conversion done in 1-2 months for a job now, you are in for a surprise. I have several friends with some good experience who have done the distance learning course and it took them anywhere between 6 to 12 months. Your efforts will depend on it.

Hope this helps.